Mix the yogurt, lemon juice, Garam Masala, and cumin in a large bowl. Add the chicken thighs to the marinade and cover all the chicken pieces with it. Cover with plastic wrap and refrigerate for 30 mins to overnight.
Remove the chicken pieces from the marinade, shaking off the excess. Heat the oil in a large skillet over medium-high heat and brown the chicken pieces for 3-4 minutes on each side. Transfer to a plate and keep warm.
Melt the butter in the same skillet over medium-high heat. Sautee the onion until translucent, about 2 minutes. Stir in the garlic and ginger and cook for 30 seconds more. Add the cumin, coriander, garam masala, chili powder, salt, pepper, and turmeric. Cook until fragrant, about 1 minute.
Stir in the chicken broth, crushed tomatoes, and sugar. Return the chicken pieces to the skillet, and simmer for 10 minutes. Pour in the heavy cream and simmer gently for 10 more minutes. Serve with freshly chopped cilantro on top and with a side of Basmati rice.