Simmer the onions on a gentle heat, in a little water, or stock from which you have skimmed the fat.
When they are done, the liquid should be reduced to a sort of clear syrup.
Add the chopped mushrooms to this, cover, and simmer for 20 minutes.
Beat up the yolk of an egg with some lemon juice in a basin, gradually beating in the warm onion and mushroom sauce, and then pour this thickened sauce onto a hot dish.
Place poached eggs on this, garnished with small pieces of fried bread.