Peel and finely chop the onions. Fry in hot fat. Deglaze with wine, bring to the boil and boil down until the bottom of the pot is only moist. Add cream, except for 2 tablespoons. Bring to the boil.
Whisk egg yolk and remaining cream. Stir into the sauce. Do not let it boil anymore. Season to taste with salt and pepper. Sprinkle with parsley strips
Variation: With cooked ham, mushrooms, nuts, herbs (basil, sage), lemon, grated cheese