In a large bowl, combine the olive oil, soy sauce, Worcestershire sauce, lemon juice, minced garlic, cumin, paprika, salt, and pepper. Add the beef cubes and toss to coat. Cover and marinate in the refrigerator for at least 30 minutes, or up to overnight.

If using wooden skewers, soak them in water for 30 minutes.

Thread the marinated beef onto the skewers, alternating with chunks of bell pepper and red onion if desired.

Preheat your grill to medium-high heat.

Place the skewers on the grill and cook for about 10-12 minutes, turning occasionally, until the beef reaches your desired level of doneness.

Remove the kebabs from the grill, let them rest for a few minutes, then serve with your favorite sides or sauces.