Prepare the Sauce. In a medium bowl, whisk together the pineapple juice (from the canned pineapple), brown sugar, soy sauce, ketchup, rice vinegar, and minced garlic. Mix until everything is fully combined. This creates the perfect sweet and tangy glaze.
Assemble the Crockpot. Add the frozen meatballs to the bottom of the crockpot. Pour the sauce over the meatballs, ensuring they are evenly coated. Add the pineapple chunks and diced bell peppers to the pot. Gently stir everything together.
Cook the Meatballs. Cover the crockpot with the lid and cook on low for 4-5 hours or high for 2-3 hours. Stir occasionally to coat the meatballs in the sauce. About 30 minutes before serving, mix 2 tablespoons of cornstarch with 2 tablespoons of water to create a slurry. Stir the slurry into the crockpot to thicken the sauce.
Garnish and Serve. Once the sauce has thickened, garnish with sliced green onions and a sprinkle of sesame seeds. Serve warm and enjoy!