Prep the slow cooker: Lightly grease the insert or use a liner to make cleanup easy.
Make the sauce: In a bowl, whisk barbecue sauce, 1/2 cup pineapple juice, soy sauce, brown sugar, apple cider vinegar, garlic, and red pepper flakes until smooth.
Add meatballs: Place frozen meatballs and pineapple chunks in the slow cooker.
Pour the sauce over the top and stir to coat everything evenly.
Cook low and slow: Cover and cook on Low for 3–4 hours or on High for 1.5–2 hours, until the meatballs are heated through and the sauce is glossy.
Thicken if needed: For a stickier glaze, remove the lid during the last 20–30 minutes, or stir in a slurry of 1 teaspoon cornstarch mixed with 2 teaspoons water and cook 10–15 minutes more.
Finish and serve: Taste and adjust salt, heat, or sweetness. Garnish with sliced green onions and sesame seeds. Serve over rice, with roasted veggies, or in slider buns.