To a bowl, add all ingredients under "chicken marinade" except chicken. Mix well. Cut three shallow slits into the chicken thighs and rub marinade all over. Cover and refrigerate for a few hours or overnight.
Cook chicken on a charcoal grill for smoky flavor. You can also cook on an indoor grill pan or a regular fry pan. This chicken can also be made in the oven.I cooked the chicken in a cast iron pan. Put a tablespoon of oil, just to cover the bottom of the pan. Cook chicken on both sides for a few minutes until golden. Then lower the heat, cover and cook for approximately 10 minutes until chicken is completely cooked. Set aside until sauce is ready. Chicken can be made the previous day and refrigerated.
For the sauce, place all ingredients under "sauce ingredients" in a pot. Stir and bring to a boil. Cover and cook for 15 minutes, stirring occasionally until tomatoes are soft. Set aside for a few minutes so it's not boiling hot when putting in the blender. Remove bay leaf and discard. Place contents into a blender and blend until smooth. For a silky smooth sauce, pour blended sauce into a strainer set over a bowl. Move a spoon or ladle back and forth to push the sauce through. Discard solids in strainer.
Final Step - Wash out and dry the same pot used to cook the sauce ingredients. Heat up the pot and pour in oil. Add coriander and ginger/garlic. Stir and add butter. Heat should be medium low so butter doesn't burn. Cook until raw taste of garlic/ginger goes away - less than a minute. Add chili powder, stir for 20 seconds. Add strained sauce and salt. Stir and bring to a boil. Add sugar to taste. Simmer for 3 minutes. Add apple cider vinegar if necessary. Taste and adjust salt and sugar. Add heavy cream to taste. Sauce should have a hint of sweetness and be a little tangy.
Stir in butter to give the sauce a beautiful glossy sheen. Add the toasted fenugreek leaves, crush it in your palms. Tear the chicken into pieces and add it to the sauce. Stir and let it heat through for 3 minutes, covered on low heat. Serve with basmati rice or naan/roti/paratha
***Use less chili powder for a very mild sauce***To Roast Kasuri Methi, place leaves in a small pan, heat for a few minutes, stirring. Turn heat off when it smells toasted. Move to a bowl and set aside.