Meatball Sliders

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.

Cover and cook on low-heat setting for 4 to 5 hours or on high-heat setting for 2 to 2 1/2 hours.

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.