Set oven rack about 6 inches from broiler and preheat broiler. Line a broiler pan with heavy-duty aluminum foil or a double layer of regular-strength foil.
In a small bowl, mix together olive oil, mustard and thyme. Thread alternating cubes of beef, pieces of bell pepper and wedges of onion onto 4 metal skewers. Brush each skewer with equal amounts of oil-mustard mixture. Season with salt and pepper. Broil, turning skewers once, until beef is no longer pink outside and onions are slightly charred, about 10 minutes. Serve with broiled tomato halves and herbed rice, if desired.