Place whole peppers on broiler rack. Place rack 6 to 7 inches from heat. Broil 3 minutes on each side or until skin begins to bubble. Remove from oven, and let cool to touch. Rinse peppers with cool water, and remove bubbled skin. Remove and discard seeds. Slice peppers into 1/4-inch-wide strips. Set aside.
Sauté onion and garlic in butter until tender; remove from heat, and add pepper strips. Toss gently to coat. Add salt to taste. Serve immediately.