Prepare the Marinade: Blend garlic, vinegar, olive oil, rosemary, and beef knuckle cubes to create the marinade.
Marinate the Lamb: Pour the marinade over the lamb, ensuring it's well coated. Let it marinate.
Prepare Vegetables: Cut onions into large pieces and prepare peppers by removing seeds and cutting into large pieces.
Add Mustard to Meat: Add mustard to the marinated lamb and mix well.
Assemble Kontosouvli: Alternate meat and vegetables on metal rods. Wrap with parchment paper and then aluminum foil.
Bake: Place on a baking sheet with a grill. Bake at 180°C for 2.5 hours, then uncover and bake for an additional 15-30 minutes until nicely colored.
Serve: Serve with souvlaki pitas, French fries, tzatziki, and lemon slices.