Heat 2 tablespoons butter in Dutch oven. Sauté onions until tender. Remove with slotted spoon. Combine flour, rosemary, and salt in bowl or plastic bag. Coat chicken with flour mixture. Melt remaining 3 tablespoons butter in Dutch oven. Brown chicken over moderate heat. Add stock, bring to a boil, reduce heat, cover, and simmer 30 minutes. Add sautéed onions, tomatoes, and corn. Cover and simmer 10 to 15 minutes longer.