Prepare the beef by cutting sirloin into 1-inch cubes and placing them in a large mixing bowl.

Mix the marinade by adding Mediterranean sea salt, minced garlic, marjoram, oregano, and parsley to the beef, followed by the juice of one lemon. Combine well.

Add olive oil to the beef mixture, stirring until all pieces are coated. Cover and refrigerate for at least one hour.

Soak bamboo skewers in water for about 30 minutes to prevent burning on the grill.

Cut bell peppers and red onion into 1-inch chunks for even cooking.

Preheat the grill to 400°F (200°C). This high temperature is essential for charring the kebabs.

Assemble the kebabs by threading marinated beef cubes, bell peppers, and onion alternately onto the skewers.

Grill the kebabs for approximately 10 minutes, turning halfway to ensure even cooking.

Serve the kebabs hot along with tzatziki sauce, fresh salad, or pita bread.