Season the pigeons with salt and pepper. Stuff the body cavities with mint leaves, and brown the birds all over in butter. Transfer to a flameproof casserole.
Fry the onion in the remaining butter in the frying pan. Pour on the wine, reduce slightly, then add the stock. Pour over the pigeons in the casserole and cook on top of the stove for about 1 hour.
Fifteen minutes before t