Pierce tomatoes with knife. Place in microwave-safe bowl; sprinkle with brown sugar. Cover loosely. Mico-cook on high (100% power), 3 minutes or until skins burst and tomatoes are soft, stirring once; set aside.
In saucepan bring 2-1/2 cups water and 1 tsp. salt to boiling; stir in polenta. Reduce heat; cook 5 minutes, stirring often.
In shallow dish combine flour with 1/2 tsp. each salt and pepper; add buttermilk to another dish. Dip chicken in buttermilk then flour. Heat 3 tablespoons cooking oil in large skillet over medium-high heat; add chicken. Cook 4 minutes per side or until no pink remains (170 degrees F); remove.
Drain pan drippings. Add spinach; cook until just wilted. Season to taste. Serve chicken with spinach, polenta, and tomato jam. Makes 4 servings.