Mushroom Soup with Roux and Egg Yolk
A delicate seasonal mushroom soup thickened with a light roux, finished with egg yolk, sour cream, and a squeeze of lemon.
Historical recipe
Modernised adaptation of an early 20th‑century source. Not independently tested by Attic Recipes. Quantities, temperatures, and food safety guidance have been updated for a contemporary kitchen — we cannot guarantee accuracy or results. Always follow current food safety guidelines for your region. If you have a health condition, allergy, or dietary requirement, consult a qualified professional before preparing this recipe.
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- Dairy
- Eggs
- Gluten
Safety note
Never use wild-foraged mushrooms unless you are an expert mycologist or the mushrooms have been identified by a certified expert. Many toxic species closely resemble edible varieties. Misidentification can be fatal.
Always use commercially cultivated mushrooms from a reputable source — button, cremini, or portobello. If using foraged mushrooms, have them verified by an expert before cooking.
Additional notes
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Warning
This soup is finished with a raw egg yolk that is tempered but not fully cooked. Not recommended for pregnant women, young children under 5, the elderly, or immunocompromised individuals.
Omit the egg yolk and increase sour cream to 3–4 tablespoons for a safe alternative with a similar creamy finish.
-
Warning
Sodium content is highly variable depending on the bone broth used. Commercial broths can contain 400–900mg sodium per 250ml serving. Total sodium per bowl may significantly exceed 800mg with a salted broth. Those managing hypertension or kidney conditions should use homemade unsalted broth.
Use homemade unsalted bone broth and season only at the end.
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Caution
Contains dairy (butter, sour cream) and egg. Not suitable for those with dairy or egg allergies.
Substitute butter with neutral oil and sour cream with a plant-based alternative for a dairy-free version; omit the egg yolk.
- 1
Fill a bowl with cold water and add a squeeze of lemon juice. Clean the mushrooms and place them in the acidulated water briefly to prevent browning. Remove and set aside.
Tip Do not soak mushrooms — a quick swish in the acidulated water is enough. Prolonged soaking waterloggs them and dilutes flavour. - 2
Place the cleaned mushrooms in a small saucepan, cover with fresh cold water, and bring to a gentle boil. Simmer for 5–7 minutes. Remove the mushrooms with a slotted spoon and slice. Reserve the mushroom cooking water — do not discard it.
Tip The mushroom cooking water holds significant flavour and is essential to the final soup. - 3
In a medium saucepan, melt the butter over medium heat. Add the flour all at once and stir constantly with a wooden spoon or whisk. Cook the roux until it is pale golden and smells faintly nutty — do not allow it to darken.
Tip For the dried mushroom variation (see below), cook the roux to a slightly deeper golden colour. - 4
Gradually pour in the bone broth, whisking constantly to prevent lumps, followed by the reserved mushroom cooking water. Bring to a gentle boil, whisking, until the soup is smooth and lightly thickened.
- 5
Add the sliced mushrooms to the soup. Reduce heat and simmer gently for 5 minutes. Season with salt, white pepper, and the juice of ¼ lemon.
- 6
In a soup bowl, whisk together the egg yolk and 2 tablespoons of sour cream. Ladle a small amount of hot (but not boiling) soup into the bowl and whisk to temper the mixture. Pour the tempered mixture back into the pot, stir well, and remove from heat immediately. Do not allow the soup to boil after adding the egg yolk.
Tip Tempering is essential — adding cold egg yolk directly to boiling soup will cause it to scramble. The soup should be around 70–75°C when the yolk mixture is incorporated. - 7
Ladle into warm soup bowls. Finish with a small swirl of sour cream and fresh parsley if desired. Serve immediately.
Nutrition Information per 1 bowl (approx. 400ml)
Nutritional values are approximate estimates and may vary based on specific ingredients used, preparation methods, and portion sizes.
Serving Suggestions
Serve in warmed soup bowls with good bread on the side. This soup does not hold well after the egg yolk is added — make only as much as will be eaten immediately. The soup base (without the egg yolk and sour cream finish) can be made ahead and refrigerated for up to two days; add the finish just before serving.
About This Recipe
This is a soup built on restraint — a small amount of mushrooms stretched into something deeply flavoured through careful technique. The mushrooms are boiled first, and their cooking water is kept and added to the finished soup alongside bone broth, doubling the mushroom presence without doubling the quantity. A light roux gives the soup body without heaviness, and a finish of egg yolk beaten with sour cream adds richness and a faint tang. A squeeze of lemon at the end lifts everything. It is a soup that tastes of the season it was made for.
Why It Works
The acidulated washing water serves two purposes: it prevents the mushrooms from oxidising and browning before cooking, and the acid helps firm the cell walls slightly, preserving texture. Boiling the mushrooms separately before adding them to the roux-thickened broth is an older technique that extracts maximum flavour from a small quantity of mushrooms — the cooking liquid becomes a secondary stock. The egg yolk tempering at the end is classic French-influenced Central European technique: the yolk emulsifies into the hot soup, adding richness and a slight gloss without curdling, provided the soup is off the boil when it is incorporated.
Dried Mushroom Variation
Dried mushrooms (porcini work particularly well) can be substituted for or combined with fresh. Soak 20–30g of dried mushrooms in warm water for 30 minutes before using; add both the rehydrated mushrooms and their soaking liquid (strained) to the soup in place of the fresh mushroom cooking water. Cook the roux to a slightly deeper golden colour to stand up to the more intense flavour of dried mushrooms.
A classic of early 20th century home cooking, preserved and adapted for the modern kitchen.
The Story Behind This Recipe
Historical Context
Mushroom soups thickened with a light roux and finished with egg yolk and sour cream appear consistently across Central European cookbooks of the early 20th century, prepared during the brief autumn mushroom season. The technique of reserving and incorporating the mushroom cooking water — rather than discarding it — reflects the practical economy of the period's home kitchen, where nothing of nutritional or flavour value was wasted.
Modern Kitchen Adaptation
No mushroom variety is specified — cultivated button or cremini mushrooms are the safest and most consistent modern choice. The egg yolk finish is traditional and gives the soup its characteristic richness; it can be omitted for a lighter version, or replaced with an additional tablespoon of sour cream.
This recipe is an independent modern adaptation developed from historical sources in the public domain. It is provided for informational and educational purposes only and does not constitute professional dietary, nutritional, or medical advice. Food preparation involves inherent risks. The reader assumes full responsibility for safe food handling, ingredient sourcing, and adherence to current local food safety guidelines. The site operator accepts no liability for outcomes resulting from the preparation or consumption of this recipe.
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