Preheat oven to 325 degrees. Set aside shallow roasting pan just large enough to hold meat or use heavy-duty foil to make close baking space for meat and set in larger pan.
Combine garlic with brown sugar, cumin and salt. Rub mixture over surface of meat.
Spread half of sliced onions in pan or on foil. Place meat on top. Scatter remaining onions over meat.
Puree chili sauce with chipotles; spoon over meat and onions. Pour ¼ cup water around meat (not on it). Cover tightly with foil.
Bake 3 hours. Remove top piece of foil; continue baking until meat is tender, 1 to 2 hours longer. Cool, then cover and refrigerate up to 3 days.
To reheat, remove fat from meat and onions. Reserve small cluster of onions for garnish. Transfer remaining onions and pan juices to processor or blender. Puree until smooth. Use sharp knife, preferably electric, to slice meat across grain into thin slices. Arrange slices in shallow casserole. Spoon on some sauce; arrange reserved onions on top. Reheat, covered, in 350-degree oven, for 35 minutes. Reheat remaining sauce separately.
Serve hot, drizzling with more sauce and passing the rest.