Gently combine lamb, garlic, onion, mustard, salt, pepper, vinegar, oregano, and rosemary with your hands and shape into 4 oblong patties. Grill for 5-6 minutes on each side and remove from heat. Allow to stand for 5 minutes. Place on pitas or flatbread and add onions, tomatoes, and Tzatziki. Fold like a taco and serve immediately. For easy serving and clean-up, wrap in aluminum foil.