Mussels a la Poulette

Prepare the mussels as for No. 1045; cut two ounces of onions into one-eighth of an inch pieces and cook them in a saucepan with two ounces of butter, not letting them attain a color, moisten with a pint and a half of veloute sauce (No. 415), and a small part of the mussel broth; reduce this sauce with mushroom parings added, strain it through a tammy, and thicken with a thickening of egg-yolks diluted with a little of the broth, and fine butter; cook this thickening without allowing it to boil, stirring it steadily; season to taste and serve.