Begin making the filling by rehydrating the dried mushrooms. To do this, boil some water and pour it over the dried mushrooms.
Leave them to soak for about 5 minutes and strain the liquid through some cheesecloth into another bowl. Reserve it to use it to make the borscht or freeze it for another recipe.
Pat the rehydrated mushrooms with kitchen paper and continue processing the rest of the ingredients.
Add the onions to a food processor and mince finely. Add it to a pan with hot olive oil and sauté until it turns golden and slightly translucent.
Meanwhile, mince the sliced button, the baby bella mushrooms and the rehydrated mushrooms in the blender, then stir into the frying pan.
Sauté everything together until the mushrooms have softened and the liquid they release has evaporated. Season to taste with salt and pepper.
When the mushroom have finished cooking, turn off the heat and add the 2 egg yolks, stirring everything vigorously to bind the filling together. Set aside to cool.
Note that you can alter the mushroom types and quantities according to your taste.