1 large red onion cut into thin wedges (1 1/2 cups)
2 12-ounce packages frozen cooked Italian meatballs (24)
1 24- to 26-ounce jar marinara or pasta sauce (about 2 1/4 cups)
1 tablespoon balsamic vinegar
1/2 teaspoon crushed red pepper
6 slices provolone cheese, quartered (6 ounces)
4 roma tomatoes, sliced
24 cocktail buns, split and toasted, if desired
1. Place onion wedges in a 3 1/2- or 4-quart slow cooker. Top with frozen meatballs. In a medium bowl combine marinara sauce, balsamic vinegar, and crushed red pepper. Pour over meatballs.
2. Cover and cook on low-heat setting for 4 to 5 hours or on high-heat setting for 2 to 2 1/2 hours.
3. Gently stir through mixture in cooker. Place a cheese and a tomato slice on the bottom of each cocktail bun. Top each with a meatball;replace bun tops.