Preheat a grill pan or cast-iron skillet over medium-high heat until very hot.
Slice the sirloin steak into uniform 1-inch cubes for even cooking.
Cut the red bell pepper, green bell pepper, and red onion into 1-inch squares.
In a small mixing bowl, whisk together the olive oil, balsamic vinegar, garlic powder, oregano, sea salt, and black pepper.
Place the beef and vegetables in a large bowl and pour the marinade over them, tossing well to coat every piece.
Thread the beef and vegetables onto metal or soaked wooden skewers, alternating for variety.
Sear the skewers in the hot pan for 3 to 4 minutes per side, turning once, until the beef is browned and the peppers are tender-crisp.
Transfer the skewers to a plate and let them rest for 2 minutes to allow the juices to redistribute before serving.