Place the onion, garlic, ginger, fennel seeds, if using, cumin, coriander,sugar, ginger, turmeric, cloves, cayenne, salt and pepper in a food processor and puree it into a paste.
Melt 4 tablespoons of the butter in a large deep skillet over medium-high heat. Add the chicken pieces and cook until the outside is golden brown but the inside is still not quite cooked, about 8 minutes in all. Stir occasionally but don’t fiddle with the pieces too often so they get browned. Transfer the chicken to a plate.
Add the pureed paste to the skillet and cook over medium heat for 2 minutes, stirring frequently, until it becomes slightly tender and very fragrant. Stir in the tomato sauce, tomato paste, broth or water, ground fenugreek, cinnamon stick and bay leaves and bring to a simmer. Add the partially cooked chicken and return to a simmer, adjusting the heat as needed, and simmer until the chicken is cooked through and the sauce has thickened a bit, about 10 minutes.
Add the cream, the remaining 2 tablespoons butter, lemon juice and cilantro and stir until the butter has melted and everything is hot and blended; don’t allow it to boil. Serve hot over rice.
Notes
Curry powder is a blend found easily in U.S. supermarkets and all around the world. It is is actually a Western invention, attached to the British Colonial government in the 18th and 19th centuries. It’s really a shortcut product designed to allow those without large spice pantries or deep knowledge of Indian food to approximate homemade spice blends.