Heat large skillet over medium heat. When pan is hot, add ghee and continue heating skillet until ghee is completely melted. Swirl and tilt skillet occasionally to distribute ghee across surface.
When ghee is melted, add minced onion to skillet. Sauté onions, stirring occasionally, approximately 5 minutes or until softened.
Add serrano chiles, minced garlic, and grated fresh ginger to skillet and immediately reduce heat under pan to medium-low. Sauté ingredients, stirring occasionally, 20 minutes or until mixture develops rich toffee color. Note: if mixture begins to dry or stick, stir in 2 to 3 drops of water as needed.
Once desired color is achieved, add garam masala and paprika. Stir well and cook 2 to 3 minutes or until fragrant. Note: continue adding water if needed to reduce drying or sticking.
When ingredients are fragrant, add tomato sauce to skillet and stir well until sauce is fully incorporated. Once incorporated, remove skillet from heat and let mixture cool slightly.
While mixture cools, preheat broiler on high, with oven rack positioned approximately 10 to 12 inches below heating element. Fit wire baking rack onto baking sheet, then place marinated chicken pieces on top of baking rack.
Place baking sheet under preheated broiler. Broil chicken, stopping to flip chicken periodically, 6 to 8 minutes or until chicken reaches 160° Fahrenheit internally. Once chicken reaches target temperature, remove baking sheet from oven and set aside.
While chicken cooks, carefully transfer slightly-cooled sauce mixture to blender. Blend mixture until completely smooth. See Notes below for tips on how to safely blend hot liquids.
Once smooth, return mixture to skillet and add bell pepper, fenugreek leaves, and salt. Stir to incorporate, then simmer sauce over medium-low heat 30 minutes, or until sauce is thick and very paste-like.
Remove skillet from heat and let cool slightly, then add heavy cream. Stir until cream is incorporated and mixture is very smooth, then return skillet to heat.
Taste sauce and adjust seasonings as desired. When satisfied with flavors, add chicken to skillet and gently stir until chicken is fully incorporated and coated in sauce.
Simmer dish only until chicken is warmed through. Divide prepared basmati rice between serving bowls, then top rice with chicken tikka masala. Garnish dish with finely chopped cilantro and serve warm with naan if desired.