If using wooden skewers, soak them in water for about 30 minutes to prevent burning.
In a mixing bowl, combine olive oil, ground cumin, smoked paprika, garlic powder, onion powder, salt, black pepper, and lemon juice. Mix well to create the marinade.
Add the cubed beef sirloin to the marinade, making sure each piece is well coated. Let it marinate for at least 15 minutes.
Thread the marinated beef cubes onto skewers, alternating with pieces of red bell pepper, green bell pepper, and onion.
Preheat the grill or grill pan over medium-high heat.
Once hot, place the skewers on the grill. Cook for about 3-4 minutes on each side, or until the beef reaches your desired doneness (internal temperature should be 145°F for medium-rare).
Remove the kebabs from the grill and let them rest for a few minutes.
Garnish with fresh parsley before serving.