In a large bowl, combine the cubed chicken, yogurt, tomato paste, garam masala, turmeric, salt, red pepper flakes, and lemon zest. Mix very well. Cover and set aside to marinate for 15-20 minutes (or overnight in the fridge).
In a large skillet, add the butter. Once melted, add the chopped onion, minced garlic, and grated ginger. Saute for 5-7 minutes, until onions are translucent and fragrant.
Add the chicken and coconut milk, stir until well combined, then cover to simmer for 15-20 minutes, or until chicken is fully cooked.
Stir in the fresh cilantro to enjoy warm with cooked basmati rice and naan.