This is a very old dish, very like a plate pie but made in a forcemeat crust instead of pastry. It is a good way to use up leftovers.
Make the forcemeat first. Mince and pound the lamb or veal (or process with the fat in an electric blender). Make it a smooth paste. Add the fat, seasonings, herbs, egg yolks and enough breadcrumbs to make a firm dough-like paste. Grease a 7½-8 inch round baking dish or pie plate. Halve the forcemeat. Roll out one half, if possible, and lay it in the dish to line it; if too crumbly, press it on the bottom and sides of the dish as when making a crumb shell. Scatter a little flour over the bottom. Spread the diced bacon over the flour, then the beef. Halve the pigeons lengthways, and remove the spines and wings. Place in one layer in the dish. Chop the sweetbreads and press them into the spaces between the birds, do the same with the asparagus tips if used, and with the mushrooms, chopped if large. Top with the sliced hard-boiled egg yolks and a little more flour. Season with pepper and salt. Cover with the second half of the forcemeat, rolled out or pressed on as before. Glaze with the egg. Bake at 350°F, 180°C, gas mark 4 for 30-35 minutes until the forcemeat crust is cooked and golden. Serve with rich gravy and green vegetable.
NOTE: The quantity of forcemeat given above can also be used to stuff 5-6 pigeons.