Indian Chicken and Yogurt Curry (Dahi Murgh)

Indian chicken and yogurt curry on brown rice plate garnished with strips of fresh mint
2 lb chicken pieces (I used boneless thighs, but you can use bone-in pieces or even chicken breast chunks)
1 large onion, chopped roughly
4 cloves garlic
2 t grated fresh ginger
1/4 C fresh mint leaves or cilantro, plus extra for garnish
1-1/2 T oil
1 t turmeric
1-1/2 t garam masala
Pinch of cayenne (optional)
1-1/2 t salt
1/2 C yogurt
1 15-oz can diced tomatoes, drained

In a food processor (easier) or blender (very tedious), grind the onion, garlic, ginger, mint or cilantro, and oil together into a paste. Heat a heavy deep skillet and cook this mixture, stirring, for about 5 minutes. Add the turmeric, garam masala, cayenne, and salt and cook for another minute. Stir in the yogurt and tomatoes and cook until the mixture is a thick puree. Then stir in the chicken pieces, coat them with the spice mixture, and simmer on low, covered, until the chicken is tender. Garnish with additional mint or cilantro and serve with rice or naan (you can substitute flour tortillas).