Grate the large onion and squeeze out its juice into a large mixing bowl. Combine with tomato paste, red pepper paste (if using), milk, olive oil, flaked chili, black pepper, and salt to create the marinade.

Cut the lamb into 2-inch cubes and add them to the marinade, ensuring each piece is coated. Cover and refrigerate for a minimum of 6 hours, preferably overnight.

Prepare the vegetables by cutting the bell pepper and red onion into matching sizes with the lamb. If using cherry tomatoes, leave them whole.

Once marinated, thread lamb cubes onto skewers, allowing room for even cooking. On separate skewers, thread the prepared bell pepper, red onion, and tomatoes.

Preheat your grill to medium-high heat and cook the lamb skewers for 14-18 minutes, rotating frequently. Grill vegetable skewers for 8-12 minutes until tender.

Allow the lamb skewers to rest for a few minutes before serving with warm pita and dips.