Prepare the marinade: In a bowl, combine olive oil, minced garlic, lemon juice, salt, black pepper, oregano, paprika, and cumin. Whisk until fully combined.
Marinate the beef: Place the beef cubes in a resealable bag or shallow dish. Pour the marinade over the beef and toss to coat evenly. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 6 hours for deeper flavor. Prepare the vegetables: While the beef marinates, wash and cut the vegetables (bell peppers, onions, zucchini, and mushrooms) into 1-inch chunks.
Preheat the grill: Preheat the grill to medium-high heat. If using a broiler, set the oven to broil and position a rack about 6 inches from the heat source.
Assemble the kabobs: Thread the marinated beef cubes onto skewers, alternating with the prepared vegetables.Leave a little space between each piece for even cooking. Grill the kabobs: Place the skewers on the preheated grill or under the broiler.Grill for 8-12 minutes, turning the skewers every 2-3 minutes to ensure even cooking. until the beef reaches your desired level of doneness (medium-rare is 130-135°F, medium is 140-145°F). Rest the kabobs: Once cooked, remove the kabobs from the grill and let them rest for a few minutes before serving. This allows the juices to redistribute, keeping the beef tender. Serve and enjoy: Serve the kabobs hot off the grill with your favorite sides, such as rice, couscous, or a fresh salad.