1. Place meat in a deep dish just large enough to hold it. Combine 1 can (1 1/2 cups) beer, onion, peel, and seasonings; pour over meat. Add a little more beer, if needed, to just cover meat. Marinate in refrigerator 1 to 2 days, turning occasionally.
2. Strain marinade, reserving solids and liquid. Place solids, carrot, and celery in bottom of a roasting pan. Place meat on top. Add a little liquid.
3. Roast in a 350°F oven 1 hour. Pour one quarter of remaining marinade liquid over meat. Continue roasting for 1 to 1 1/2 hours more, basting occasionally with drippings and more marinade, until meat thermometer registers 1 70°F.
4. Mix flour and 1/3 cup marinade or beer to a smooth paste. Place roast on a platter; keep warm. Skim fat from cooking liquid. Strain, pressing solids; add flour mixture and 1/2 cup beer plus water, if needed, to measure 2 cups total liquid. Cook in roasting pan or a saucepan, stirring constantly, until thickened. (For 3 cups gravy, use more beer and water plus 6 tablespoons flour.)