Cut each chicken breast half in half horizontally, and lightly coat chicken with cooking spray. Sprinkle Italian seasoning, garlic powder, black pepper, and 1/8 teaspoon salt over chicken. Heat oil in a large nonstick skillet over medium-high heat. Add chicken; cook 2 minutes on each side or until browned. Reduce heat to medium; add water. Cook 10 minutes or until chicken is done. Remove chicken from pan; keep warm.
Return pan to medium-high heat. Add red onion; sauté 2 minutes. Add bell pepper, green onions, and remaining 1/8 teaspoon salt; sauté 2 minutes or until tender.
Place bottom half of focaccia on a baking sheet; top with chicken. Arrange onion mixture evenly over chicken; sprinkle with cheese. Broil 2 minutes or until cheese melts. Top with remaining bread; slice into 4 equal portions.