1. Prepare spinach as label directs; drain well. Toast English muffins; keep warm.
2. Preheat broiler if manufacturer directs. Into bowl, tear bread into small pieces; add spinach, ground beef, egg, salt, and onion; mix well. Shape mixture into four 1-inch-thick round patties. Place patties on rack in broiling pan.
3. Broil patties about 5 minutes on each side for medium or until of desired doneness. Just before patties are done, cut mozzarella cheese into 4 slices. Top each meat patty with a cheese slice; broil 1 minute longer or until cheese melts.