Place brisket in heavy duty aluminum foil and sprinkle with 2 Tbsp liquid smoke. Wrap and refrigerate overnight. The next day open the foil, sprinkle on the chopped onions, garlic and pepper.
Rewrap and place in baking pan. Bake at 325° F for 4-5 hours, until tender when pierced with a knife. Combine the remaining teaspoon liquid smoke, ketchup, water, brown sugar, Worcestershire sauce, mustard, celery seeds and margarine. Simmer, uncovered, 30 minutes.
Stir occasionally. Slice the brisket thinly. Rewrap with the onions and sauce. Bake at 350° F 30 minutes more.Servings: