Heat oil in a nonstick skillet over medium heat. Add onion; cook 10 minutes, stirring frequently. Add bell pepper and black pepper; cook 3 minutes or until bell pepper is crisp-tender.
Coat top sides of bread with cooking spray. Top each slice with 2 ounces beef, 1/4 cup onion mixture, and 1 cheese slice.
Place sandwiches on a baking sheet coated with cooking spray; broil 2 minutes or until cheese melts.