Heat oil in a frying pan over medium-high heat. Add onions, and sauté for 3-4 minutes or until browned. Add chilies. Sauté for a minute. Add garlic cloves. Sauté for 2-3 minutes or until aromatic.
Add tomato purée, stir, and cook for 2 minutes. Stir in cooked chicken tikka (or leftover chicken). Add garam masala and chaat masala. Cook, stirring for 3 minutes. Add tomatoes and cucumbers. Stir. Cook for another 2 minutes. Stir in coriander (cilantro) leaves. Season with salt.
Garnish with more coriander leaves, sliced chilies, chopped cucumber and tomatoes. Serve with lemon wedges.