Stir yogurt and tandoori masala powder in a large bowl until combined; add chicken and toss to coat. Marinate in the refrigerator for 1 hour. Drain and discard excess marinade.

Puree onion, ginger, water, and garlic in a blender until it forms a smooth paste; set aside.

Heat 1 tablespoon oil in a large skillet over medium heat. Add chicken and cook until lightly brown, about 5 minutes. Remove from the skillet and set aside.

Heat remaining 2 tablespoons oil in the same skillet. Fry onion paste until moisture is mostly evaporated, about 3 minutes. Sprinkle garam masala and chili powder over mixture; cook for 1 minute more. Add tomato sauce, reduce heat to medium-low, and cook another 5 minutes.

Return chicken to the skillet. Add half-and-half and bring to a boil. Add butter, fenugreek leaves, and salt; reduce heat to low and simmer, uncovered, until chicken is no longer pink in the middle, 15 to 20 minutes.