To make the curry paste. In a blender or food processor, combine the onion, shallot, garlic, ginger, garam masala, turmeric, salt, red pepper flakes, and lemon zest. Pulse until a smooth paste forms, about 1 minute.To make the chicken. In a gallon-size zip-top bag, combine 2 tablespoons of the curry paste, the chicken, and the yogurt. Seal the bag and massage the mixture into the chicken to cover completely. Marinate at room temperature for 30 minutes or refrigerate up to overnight.Heat a large, high-sided skillet over medium heat. Add ¼ cup to 1/2 cup (use to your taste) of the curry paste and cook until fragrant, about 1 minute. Stir in the chicken, coconut milk, and tomato paste. Cover and cook until the chicken is cooked through, 15 to 20 minutes. Remove the lid and simmer until the sauce thickens slightly, about 5 minutes more.Stir in the cilantro and serve the chicken over rice.