For the marinade:
In large bowl, mix the yogurt, ginger, garlic, salt, and black pepper. Poke chicken with a fork, then add to marinade. Fold to cover and marinate at least 30 minutes, at most refrigerated overnight.
For the sauce:
When you're ready to make the curry, place a large skillet over medium heat, and add the butter and olive oil. When butter has melted, add the cumin seeds, red crushed pepper, garlic, ginger, and serrano peppers. Sauté about 2 to 3 minutes. Add the onions. Sauté until lightly browned around the edges. Add the tomato paste and cook until the tomato has darkened in color, about 2 to 3 minutes. Add the spices, and sauté for about 1 minute to draw out their flavors. Add the tomatoes, salt, and water. Bring to a boil, then turn down the heat to medium low, and cover. Cook for 20 minutes. Take the pan off the fire, and allow the sauce to cool for 5 minutes.
Meanwhile, turn oven to 425 °F. Pull the chicken breasts out of the marinade and place on the cooking sheet. Place in the oven, and cook for 20 to 30 minutes. Cut cooked chicken into large bite-size chucks. Do not worry if the chicken is still a little uncooked, it will finish cooking in the sauce.
Pour sauce into a blender or food processor, and process until smooth. Pour back into the pan, and bring back up to a boil. Add the chicken and fenugreek leaves, if using. Reduce heat to a simmer, and cook, covered, for about 10 minutes. Add cream and stir through. Serve over rice, with naan.