Curry, without yogurt and cashews, can be made 5 days ahead and cooled completely, uncovered, then chilled, covered. Reheat over low heat before stirring in yogurt and ground cashews.

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

Heat butter in a 5- to 6-quart wide heavy pot over moderately low heat until foam subsides, then cook onions, garlic, and ginger, stirring, until softened, about 5 minutes. Add curry powder, salt, cumin, and cayenne and cook, stirring, 2 minutes. Add chicken and cook, stirring to coat, 3 minutes. Add tomatoes, including juice, and cilantro and bring to a simmer, then cover and simmer gently, stirring occasionally, until chicken is cooked through, about 40 minutes. (If making ahead, see cooks' note, below.)

Just before serving:
Pulse cashews in a food processor or electric coffee/spice grinder until very finely ground, then add to curry along with yogurt and simmer gently, uncovered, stirring, until sauce is thickened, about 5 minutes.