In large microwavable bowl, place meatballs. Microwave on High 3 minutes. Cut meatballs in half; return to bowl. Stir in marinara sauce. Cover with waxed paper. Microwave on High 6 to 8 minutes, stirring once or twice, until hot.
Meanwhile, set oven control to broil. Cut bread in half crosswise; cut each in half lengthwise, making 4 pieces. Place bread, cut side up, on ungreased large cookie sheet. If desired, toast lightly under broiler.
Spoon hot meatballs and sauce evenly onto bread. Sprinkle with cheese. Broil 4 to 6 inches from heat 1 to 2 minutes or until cheese is melted and bubbly. To serve, cut each piece in half.