If using left-over chicken or turkey, chop it finely and set aside. Continue with the recipe from Step 3, using stock to make the sauce rather than water, and add the chicken or turkey to the sauce at Step 6.
If using veal or beef, season with salt and pepper to taste.
Place a large pan over medium-high heat and melt 2 tablespoons of butter. Add the veal or beef, wine, onion, clove, bay leaves, nutmeg or mace, parsley, thyme, lemon zest or juice and water. (Use chicken stock if you have chosen to use chicken or turkey.) Bring to a boil, then reduce heat and simmer for 45 minutes to an hour. By this time the meat should be tender (or if using poultry, the stock will be bursting with flavor).
Strain the stock and reserve, along with the meat. Finely chop or cut the meat into into small pieces.
Melt the remaining butter in the pan, stir in the flour and keep stirring for a few minutes on low heat. Don't let it brown. Slowly and gradually add the stock and cook over moderate heat, stirring until the sauce is smooth and thick. Add more flour, or corn starch to thicken the sauce if necessary. Cool slightly and add the 3 egg yolks and mix well (reserve the whites for later).
Add the veal, beef, chicken or turkey, mix well and season to taste. The mixture should be thick and stiff by this time.
Set the mixture aside to cool thoroughly. When ready, cut or separate the stiff, thick mixture into rolls about 1.5" thick and about 3" long.
Spread the crumbs on a clean, dry chopping board. Slightly beat the eggwhites in a deep plate until just incorporated. Roll the croquettes through the breadcrumbs, then through the egg whites and again through the crumbs. (Make sure that the second coating of crumbs is even and thick and no meat mix sticks out, otherwise the croquettes may burst when being deep fried).
Deep fry the croquettes in batches of four or five for about 4 minutes, or until they are golden brown. Drain on absorbent paper.
Serve hot, with French fries or fresh chunks of bread. The best way to season is to slather with mustard, as the Dutch do!