In a medium bowl, whisk together pineapple juice (from canned pineapple), mustard, ketchup, Worcestershire sauce, vinegar, and brown sugar.
Place pineapple chunks and bell peppers in a 7-quart slow cooker. Cover with sauce and cook on low for 3 to 4 hours.
About 30 minutes before sauce is done cooking, combine ground beef, egg, breadcrumbs, and garlic powder in a medium bowl. Mix thoroughly and shape into 18 meatballs.
In a large, nonstick skillet over medium-high heat, brown meatballs on all sides. Add a little bit of oil to the pan if the meatballs start to stick.
Place meatballs in slow cooker with the sauce, fully covering the meatballs. Turn slow cooker up to high and cook 30 minutes, or until meatballs are cooked through.
Serve over rice and top with diced green onions.
Frozen, pre-made meatballs can be used in place of the first four ingredients, in which case the cook time after adding meatballs can be reduced. For a healthier version, switch to a low-sodium ketchup and cut the brown sugar by half.
Per serving: 821 calories, 10 grams fat (3.5 grams saturated fat), 134 grams total carbohydrates, 3 grams fiber, 47 grams protein.