In a Dutch oven, melt 4 tablespoons butter over medium-high heat. Add the onions, and cook until translucent, about 4 minutes. Stir in the flour, and cook over low heat, without browning, for 4 minutes. Whisk in the stock until smooth and combined. Add the parsley, bay leaf, and thyme. Simmer for 20 minutes.
In a large skillet, heat the remaining 4 tablespoons butter over medium heat. Add the mushrooms, stirring to coat with butter. Add the salt and lemon juice, and cook 5 minutes.
Add the cooked mushrooms to the stock mixture, and simmer 5 minutes.
In a small bowl, whisk the egg yolks and cream until combined. While whisking constantly, slowly add 1 cup of hot soup into the cream mixture. Add to the soup, and heat until warmed through. Be sure not to let the soup get too hot or the egg yolks will curdle. Season with salt.
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