Heat a 12-inch nonstick skillet over medium heat. Coat sausage with cooking spray; add sausage to pan, and cook 3 minutes on each side or until browned. Remove sausage from pan; keep warm. Coat pan with cooking spray; add onion. Cook 8 minutes or until browned, stirring frequently; add water and sugar, stirring well. Place sausage over onion mixture; reduce heat to low. Cover and cook 5 minutes.
Spread 1 1/2 teaspoons mustard on each slice of bread. Top each with about 1/4 cup onion mixture and 2 sausage halves. Serve immediately.