Melt the butter in a casserole and sauté the onions. In a pan, bake the pork cheeks seasoned with pepper and salt until golden brown on all sides. Add the meat to the onions in the casserole. Deglaze the pan with the beer and remove the baked items from the bottom of the pan. Pour the warm beer with the veal stock over the meat, add the bouquet garni and put on the slice of bread coated with mustard. Stew on low heat for about 2 hours.
Take the meat out of the pot and reduce the sauce a little more if necessary.