1. Brown pork loin well on all sides in kettle or Dutch oven; remove from pan. (Or brown in a large, heavy, roasting pan on surface burners.)
2. Drain all but 3 tablespoons drippings from pan. Saute onions and carrots until soft in pork drippings. Stir in beer, salt, pepper, bay leaf and whole cloves. Return pork to kettle and cover. (If using roasting pan, cover tightly with aluminum foil.)
3. Braise in moderate oven (350°) for 2 hours, or until pork is tender when pierced with a 2-tined fork. Place pork on platter and keep warm.
4. Pour cooking liquid from kettle into a large bowl. Skim off fat; remove bay leaf. Place liquid and solids in container of electric blender; cover and whirl at low speed until smooth (or press through a sieve). Pour sauce into saucepan. Bring to boiling, stirring often. Stir in a little gravy coloring, if you wish. Spoon the sauce generously over the pork after it has been sliced.