In a large skillet, heat oil over medium-high heat. Brown chicken in two batches, about 8 minutes per batch. Transfer to a plate. Add onion, garlic and rosemary to pan; cook 4 minutes, until onion is softened. Add wine and bring to a boil, stirring to loosen any browned bits. Add salt and red-pepper flakes.
Return chicken, skin side up, and accumulated juices to skillet. Boil until almost all the wine has evaporated, about 2 minutes. Add tomatoes. Cover, reduce heat to low and simmer 30 minutes, until chicken is cooked through.
Transfer chicken to a serving platter. Boil sauce 2 minutes to thicken; spoon over chicken.
