Combine olive oil, soy sauce, lemon juice, honey, smoked paprika, cumin, garlic powder, salt, and pepper in a small bowl. Whisk until the honey dissolves completely—this takes about 30 seconds and matters because undissolved honey creates hot spots on the grill.
Place beef cubes in a large zip-lock bag or glass bowl, then pour the marinade over them. Seal or cover, then refrigerate for at least 15 minutes but no more than 8 hours. I confess I've forgotten marinating overnight once—the texture became almost mushy, so 6 hours maximum is my hard rule now.
Thread beef, red peppers, green peppers, and onion onto metal skewers, alternating vegetables and meat for even cooking. Push pieces firmly together so they don't spin when you turn them. Leave about 1/4 inch of space between pieces so heat reaches all sides and creates that light char.
Preheat your grill to medium-high (about 400°F) for 10 minutes. Brush grates with oil using tongs and a paper towel—this prevents sticking and lets you develop those gorgeous char marks. I always check that the grill is ready by holding your hand 6 inches above the grate; you should only tolerate the heat for 2-3 seconds.
Lay kabobs across the grill grates perpendicular to them so they don't roll. Cook for 8 to 10 minutes on the first side without moving them—this is where the magic happens. Patience here creates those caramelized edges on your grilled beef kabobs pepper onion that people actually remember.
Turn kabobs once and cook another 8 to 10 minutes until beef reaches 130°F internal temperature for medium-rare. Use a meat thermometer inserted into the thickest beef piece, not touching the marinade or a vegetable. Sandra always tells me to trust the thermometer over the look, and she's right—the meat continues cooking 5 degrees after you remove it.
Transfer grilled beef kabobs pepper onion to a clean plate and let rest for 3 minutes before serving. This resting period allows the juices to redistribute so every bite stays tender instead of dry. Serve immediately while the grill marks are still visible.