SPICE MIX: Combine the 2 tablespoons cumin, 2 tablespoons garam masala, 1 and 1/2 tablespoons turmeric, 1 teaspoon paprika, 1 and 1/2 tablespoons finely minced ginger (peel the ginger with a vegetable peeler or scrape off the peel with the sharp edge of a spoon), 1 and 1/2 tablespoons finely minced garlic, and 1/8 teaspoon cayenne pepper (See Note 1). Stir together and set aside.
MARINATE CHICKEN: Trim the fat and cut the chicken into 1-inch pieces and place in a large bowl or bag. Add the 3/4 cup yogurt, 1 tablespoon lemon juice, 1/2 teaspoon salt, and HALF of the spice mix (just eyeball it). Stir to coat (or knead the bag to coat). Cover the chicken tightly (or seal the bag) and refrigerate for at least 30 minutes up to overnight. Do not marinate longer than 24 hours.
COOK ONION AND SPICES: Add the 2 tablespoons butter to a large skillet and heat over medium heat. Add in the 1 and 1/2 cups finely diced onion and cook, stirring occasionally for 7-10 minutes. Reduce the heat if it begins to burn. Add in the reserved half of the spice mix and 1 tablespoon tomato paste. Stir for 2-3 minutes or until very fragrant. Reduce heat if the mixture begins to burn.
COOK CHICKEN: Push the onion and spices to the edges of the pan and add the chicken in the middle of the pan (remove the chicken out of the marinade but don't wipe off the marinade from the chicken (but also don't pour the marinade left in the bag/bowl into the pan)). Cook for about 3-4 minutes, over medium high heat, or until the chicken is white on the outside (but not cooked through). Stir often and incorporate the spices and onion into the chicken as you cook. (See Note 3)
SAUCE: Once the chicken is white all over, add in the can of fire-roasted crushed tomatoes, 1 and 1/4 cup heavy cream, 1 tablespoon white sugar, and add salt to taste (about 1 teaspoon). Stir, and reduce the heat to low. Simmer 15-20 minutes or until chicken is cooked through (165 degrees F) and sauce has thickened. If desired, stir in some chopped cilantro here (about 1/3 cup).
SERVE: Serve curry over cooked basmati rice (See Note 2) and garnish with additional cilantro leaves (optional). Serve with flatbread, roti, or naan.