1/2 stick (1/4 cup) unsalted butter
2 medium onions, finely chopped (2 cups)
2 large garlic cloves, finely chopped
1 tablespoon finely chopped peeled fresh ginger
3 tablespoons curry powder
2 teaspoons salt
1 teaspoon ground cumin
1/2 teaspoon cayenne
1 (3 1/2- to 4-pound) chicken, cut into 10 serving pieces
1 (14.5-ounce) can diced tomatoes
1/4 cup chopped fresh cilantro
3/4 cup cashews (1/4 pound)
3/4 cup plain whole-milk yogurt
Accompaniment: cooked basmati or jasmine rice
Garnish: chopped fresh cilantro