Make 1/4-inch-deep cuts in ham; rub ham with salt and pepper, and place, fat side up, in a roasting pan. Stud onion with cloves, and place in pan with celery and garlic.
Bake at 325 for 3 to 3 1/2 hours or until a meat thermometer inserted into the thickest portion registers 160 (about 20 minutes per pound), basting with wine.
Remove ham from pan. Skim fat from drippings.
Whisk together drippings, broth, brandy, and flour in a saucepan. Cook over medium heat 5 minutes or until thickened.