Chicken Tikka Masala

In a large bowl, mix the yogurt, lemon juice, 2 tsp garam masala, cumin, paprika, half the garlic, and half the ginger; add the chicken chunks and stir to coat fully.
Cover and marinate in the refrigerator for at least 2 hours, or up to overnight, so the flavors penetrate the meat.
Heat a large skillet or grill pan over high heat until very hot, then add the marinated chicken in a single layer, working in batches if needed.
Sear the chicken pieces for 3 to 4 minutes per side until charred spots form and the outside is opaque, then remove to a plate (it does not need to be fully cooked yet).
In the same pan, reduce heat to medium and melt the butter, then add the chopped onion and cook for 5 minutes, stirring, until softened and translucent.
Add the remaining garlic and ginger and cook 1 minute until fragrant, stirring constantly so it does not burn.
Stir in the remaining 1 tsp garam masala and cook 30 seconds to bloom the spices, then pour in the crushed tomatoes and salt.
Simmer the sauce over medium-low heat for 15 minutes, stirring occasionally, until it thickens and deepens in color.
Stir in the heavy cream and return the seared chicken and any accumulated juices to the pan.
Simmer gently for 8 to 10 minutes, uncovered, until the chicken is cooked through to 165F and the sauce coats the back of a spoon.
Taste and adjust salt, then stir in most of the cilantro, saving some for garnish, and serve hot over rice with the remaining cilantro on top.