Whisk together the olive oil, balsamic vinegar, and mustard in a shallow dish.
Season the steak with salt and pepper, then toss in the marinade.
Let marinate for 30 minutes, then slide onto skewers with the peppers.
Preheat a grill pan to high heat and grease with cooking spray.
Cook the kebabs for 2 to 3 minutes on each side until the beef is done.