Revised recipe (original here)
2 cups buttermilk
2 1/2 Tbsp spiced onion
2 1/2 Tbsp ketchup
2 1/2 Tbsp chopped cilantro
1 tsp table salt (omit if you’re using a kosher chicken)
1 fryer chicken, cut into 8 pieces (3 to 3.5 lbs)
2 Tbsp vegetable oil (plus more, if needed)
1 onion, sliced
2 Tbsp water (optional)
3 cloves garlic, minced
1 1/4 tsp fresh ginger, minced
1 1/4 tsp ground cumin
2 1/2 tsp ground coriander
3/4 tsp turmeric
1 tsp cayenne pepper (add more or less to your taste)
1/4 tsp garam masala
2 1/2 tsp AP flour
1 14.5-oz. can diced tomatoes, drained
Combine the buttermilk, spiced onion, ketchip, cilantro, and salt in a large bowl. Add the chicken pieces, rubbing the mixture into the skin. Cover the bowl and refrigerate overnight or for at least 5 hours.
Remove the chicken pieces from the marinade. Use your fingers to get some of the extra liquid off. Reserve the marinade. Heat the oil over medium-high in a large pan or pot (the pan I used was 14″ in diameter and about 3″ deep). When the oil is shimmering, add the chicken pieces skin-side down. Cook for 3-4 minutes per side, or until golden brown. Remove the chicken from the pan and set aside.
Lower the temperature to medium, and add more oil to the pan if it looks too dry. Add the onions and cook for 5 minutes, scraping up the fond and stirring contantly. If you’re worried things are going to burn, add the water (probably won’t need it if you’re using a nonstick pan) to deglaze. Next, add the garlic and ginger. Stir and cook for 1 minute.
Switch to a whisk. Add the cumin, coriander, turmeric, cayenne, garam masala, and flour to the pan. Whisk constantly for 2-3 minutes. Add the reserved marinade to the pan, continuing to whisk so there won’t be any lumps. Add the tomatoes (you can switch back to a wooden spoon or spatula at this point). Bring to a boil, then reduce to a simmer.
Add the chicken pieces and any accumulated juices to the pan. Cover the chicken pieces with the sauce, then cover the pan and slowly simmer for 45 minutes to 1 hour, or until the chicken in tender. Taste and adjust seasonings. Serve over rice.