Combine bell pepper, onion, garlic, avocado, mango and lime juice in a small bowl. Stir in cilantro and season with salt and pepper. Set salsa aside.
Prepare a charcoal fire, let burn to a gray ash and set grill about 6 inches above coals. (Or preheat broiler and set broiling rack 6 inches from heat source.)
Rub steak with oil and season both sides with salt and pepper. Grill or broil steak, about 6 minutes per side for medium-rare. Remove steak to a work surface, cover loosely with foil and let sit for 10 minutes. Slice steak very thin on diagonal and serve with salsa.