1. Cut chicken into 1-inch chunks. Sprinkle with ginger-garlic paste and tandoori spice mix (or paprika and garam masala). Season with a sprinkle of salt, then toss together until chicken is well-coated. Cover and set aside for 15 minutes at room temperature, or up to overnight in the fridge.
2. Place large skillet over medium heat and add 2 tablespoons oil plus 1 tablespoon butter. Once butter has melted, add the cumin seeds and bay leaves. Cook until seeds are slightly darkened in color.
3. Add onions, and sauté until softened and *just* starting to turn golden brown.
4. Meanwhile, as onions cook, grab another skillet. Warm remaining 2 tablespoons of oil over medium high heat until shimmering. Add enough chicken to form an even layer. Cook, stirring occasionally until chicken browns. Remove from pan and cook additional batch of chicken if necessary. There should be a dark coat of spices stuck to the bottom of the pan. Don't worry! Add 1/4 cup water and apple cider vinegar to pan and bring to a boil, scraping at the solids with a wooden spoon. Then turn the heat off and set aside.
5. Once the onions are softened, turn heat down and add paprika and turmeric. Stir well and cook for no more than 1 minute.
6. Add tomatoes and their juices, 2 tablespoons ginger garlic paste, cashews, salt, garam masala, sugar and vinegar mixture from the other pan. Stir together, bring to a boil over high heat. Then turn heat down low and simmer, uncovered, for 15-20 minutes until thickened.
7. Pull out bay leaves. Purée tomato mixture in a blender with 1/4 cup water. Return to pan. Add chicken and any accumulated juices. Add fenugreek leaves. Stir together and cook over medium low heat until chicken is warmed and cooked through.
8. Turn heat to low. Stir in cream and a little extra water if the gravy seems too thick for your liking. Turn heat off completely and add cold butter, stir slowly to enrich the sauce with that buttery goodness. Taste for salt and garnish with a drizzle of extra cream and a delicate sprinkle of extra fenugreek leaves.