Preheat the grill to high heat, about 450°F to 550°F.
In a large bowl, combine the cubed top sirloin, olive oil, and steak seasoning; toss until the meat is evenly coated. Set aside to marinate briefly while you prepare vegetables.
Prepare the vegetables: peel and cut the red onion into 1-inch pieces, remove mushroom stems, and seed and cut the red bell pepper into 1-inch pieces.
Thread the marinated beef and vegetables onto skewers, leaving a little space between pieces so heat can circulate; alternate beef and vegetables as desired.
Place the skewers on the hot grill and cook, turning to brown all sides, about 2 to 3 minutes per side (roughly 8–12 minutes total) for medium-rare to medium, or longer to reach your preferred doneness.
Remove the kebabs from the grill and let rest for 3 to 4 minutes. If using, squeeze the juice of 1/2 lemon over the kebabs before serving.