Remove the stalk from the artichokes, and trim the tips of the leaves. Boil in salted water till tender, but not too soft. Remove from the water, drain thoroughly by standing them on a dish upside down for a few minutes. Then carefully remove the centre leaves and, with a small sharp knife, remove the choke. Fill with the following purée: Boil 3 or 4 large sliced onions till tender. Put 2 tablespoons of butter in a saucepan and, when melted, stir in 2 tablespoons of flour, and simmer till the mixture is slightly yellow. Rub the onions through a sieve and add them to the butter and flour. Stir well, add the yolks of 2 eggs, and 1 tablespoon of butter. Stir for a few minutes longer, and fill each artichoke with the purée. Sprinkle with grated cheese, and put in the oven for 15 to 20 minutes. Serve with croûtons of fried bread.