In a small bowl combine garam masala, turmeric, cumin, paprika, coriander, cayenne pepper, salt, black pepper.
In a medium bowl, whisk yogurt, lemon juice, garlic, ginger and half of the spice mixture together. Add chicken and toss to coat. Cover and chill at least 4 hours or overnight.
Meanwhile, heat oil in a large, heavy bottomed pot over medium heat. Add onion and cook, stirring occasionally until lightly browned in spots, about 5 to 8 minutes. Add tomato paste, garlic, ginger, serrano peppers, and remaining spice mixture. Cook, stirring frequently until spices are fragrant and tomato paste has started caramelizing on the bottom of the pot, about 2 minutes.
Add tomatoes, yogurt, heavy cream and 1 cup water. Scraping up any bits from the bottom of the pot. Bring to a simmer and reduce heat to medium-low and cook, stirring occasionally until flavors meld and the sauce is thickened slightly, 20 to 25 minutes. Remove from heat and cover until chicken is ready (or after cooling, refrigerate if marinating chicken for more than 4 hours).
Once chicken has finished marinating, turn on the broiler and place oven rack about 6 inches away from heat source.
Line a rimmed baking sheet with aluminium foil and place a wire rack inside the pan. Remove chicken from the marinade, scraping off any excess, and place the pieces on the rack. Broil just until lightly charred in spots, 5 to 7 minutes on the first side, before flipping pieces and broiling an additional 2 to 3 minutes.
Bring tomato sauce to a simmer if it is not already. Mix in remaining marinade. Add chicken. Continue to simmer until chicken is cooked through and sauce is thickened nicely, 10 to 15 minutes.
Divide chicken among four bowls and top with cilantro. Serve with basmati rice or naan for dipping.