Season steak with salt and pepper. Heat barbeque or indoor grill to medium-high heat. While this occurs, heat oil in a large skillet. Add peppers, onions and garlic. Season with salt and pepper. Cook, stirring, until firm-tender. Add vinegar and parsley, adjust seasoning, and then reserve on low heat.
Grill steak to desired doneness; allowing 2 to 3 minutes per side for rare, 3 to 4 for medium rare, and 4 to 5 for medium. Divide pepper mixture on 4 plates. Top peppers with steaks.