Combine ribs, sliced onion, and water to cover in a large Dutch oven. Bring to a boil. Reduce heat; cover and simmer 45 minutes or until ribs are tender. Drain and set aside.
Sauté chopped onion and garlic in butter in a medium saucepan until tender. Stir in catsup, vinegar, Worcestershire sauce, sugar, mustard, salt, pepper, and lemon juice. Bring to a boil. Remove from heat.
Place ribs on grill 4 to 6 inches from medium coals. Cook 20 to 30 minutes, turning and basting frequently with sauce.