In a large mixing bowl, whisk together the olive oil, lemon juice, red wine vinegar, minced garlic, Dijon mustard, Worcestershire sauce, dried oregano, salt, black pepper, and red pepper flakes (if using) until well combined.

Add the cubed beef to the marinade in the bowl. Toss to ensure all pieces are thoroughly coated. Cover the bowl and refrigerate for at least 2 hours, or preferably 4-12 hours, to allow the flavors to penetrate and tenderize the beef.

If using wooden skewers, soak them in water for at least 30 minutes prior to assembling the kabobs to prevent burning on the grill. While the beef marinates and skewers soak, prepare your vegetables by cutting them into uniform 1-inch pieces.

Preheat your grill to medium-high heat (about 400-450°F / 200-230°C). Lightly oil the grill grates to prevent sticking.

Thread the marinated beef cubes and prepared vegetables (bell peppers, red onion, cherry tomatoes if using) alternately onto the skewers. Try to distribute the ingredients evenly.

Place the assembled kabobs on the preheated grill. Grill for 8-12 minutes, turning them every 2-3 minutes, until the beef is cooked to your desired doneness (135°F / 57°C for medium-rare, 140°F / 60°C for medium). The vegetables should be tender-crisp with nice char marks.

Once cooked, remove the kabobs from the grill and let them rest for 5 minutes before serving. This allows the juices to redistribute, ensuring maximum tenderness.