Prep the slow cooker: Lightly grease the insert or use a liner to make cleanup easy.
Combine the sauce base: In a bowl, whisk ketchup, brown sugar, vinegar, soy sauce, garlic, ginger, and optional red pepper flakes until smooth.
Thicken the sauce: In a separate small bowl, whisk cornstarch with the reserved 1/2 cup pineapple juice until no lumps remain.
Stir this slurry into the sauce base.
Layer the ingredients: Add frozen meatballs to the slow cooker. Scatter bell pepper, onion, and pineapple chunks over the top. Pour the sauce evenly over everything and stir to coat.
Cook low and slow: Cover and cook on Low for 4–6 hours, or on High for 2–3 hours, until the sauce is glossy and thick and meatballs are heated through.
Finish and taste: Give it a good stir.
If the sauce is too thick, splash in a bit of water or more pineapple juice. If it needs more brightness, add a teaspoon of vinegar. For more sweetness, add a teaspoon of brown sugar.
Garnish and serve: Top with green onions and sesame seeds.
Serve over rice or noodles, or set out with toothpicks for a party.