This recipe is adapted from Martha's first book, Entertaining.
Cook onions in 4 tablespoons butter in a large stockpot over medium heat until soft, 4 to 5 minutes. Stir in flour and cook over low heat for 4 minutes without browning. Add the stock, blending it completely with the flour. Add the parsley, bay leaf, and thyme, and simmer for 20 minutes. Strain into a second pot.
In a large skillet, heat remaining 4 tablespoons butter over medium heat. Add the cremini mushrooms and season with salt. Cook until softened, and then add the shiitake and oyster mushrooms. Season with salt and lemon juice and cook until mushrooms are deep golden brown, 8 to 10 minutes.
Reheat stock mixture to a simmer. Whisk the egg yolks and cream until blended. While whisking, slowly add a cupful of the hot soup into the cream mixture and add back to the soup and heat to just below a simmer (if the soup simmers, the yolks will curdle). Add the cooked mushroom mixture and stir to combine. Serve garnished with parsley.