Rinse ribs and pat dry; trim off and discard excess fat. In a 9- by 13-inch casserole, mix garlic, tomato paste, tequila, and tomato juice; turn ribs over in sauce to coat well. Cover airtight and chill at least 1 hour; if marinating up to 1 day, turn ribs over several times.
Uncover ribs, turn over in sauce, and sprinkle with onions and chilies. Seal casserole tightly with foil.
Bake in a 350° oven until meat is very tender when pierced, 2 1/4 to 2 1/2 hours. Skim off and discard fat; season meat to taste with salt and pepper.