Prep the crockpot: Lightly grease the insert or use a liner to make cleanup easy.
Add the base: Place the meatballs, bell pepper, onion, and pineapple chunks in the crockpot.
Make the sauce: In a bowl, whisk together ketchup, brown sugar, rice vinegar, soy sauce, garlic, ginger, and optional sriracha.
Thicken it: In a separate cup, whisk cornstarch with the 1/2 cup pineapple juice until smooth, then stir into the sauce mixture.
Combine: Pour the sauce over the meatballs and veggies, tossing gently to coat everything evenly.
Cook low and slow: Cover and cook on Low for 4–6 hours or on High for 2–3 hours. The sauce should be glossy and thickened, and the vegetables tender-crisp.
Finish: Stir well.
If the sauce seems too thick, add a splash of water; if too thin, uncover and cook another 15–20 minutes on High.
Serve: Top with green onions and sesame seeds. Serve over steamed rice, cauliflower rice, or noodles.