Preheat the oven to 400°F and line a large baking sheet with parchment paper.
Combine the ground beef, breadcrumbs, egg, milk, onion, garlic, salt, and pepper in a large bowl.
Shape the mixture into approximately 32 evenly sized meatballs. Arrange them on the prepared baking sheet.
Bake for 12 to 15 minutes, until lightly browned and firm enough to transfer.
Whisk together the ketchup, brown sugar, vinegar, pineapple juice, soy sauce, and ginger.
Place the browned meatballs into a lightly greased 6-quart slow cooker. Pour the sauce over the top.
Cover and cook on LOW for 3 to 4 hours or HIGH for 2 to 2½ hours, until the meatballs reach 160°F.
During the final 45 to 60 minutes of cooking, add the pineapple chunks and bell peppers.
Mix the cornstarch with cold water. Stir into the sauce, cover, and cook on HIGH for another 15 to 20 minutes until thickened.
Gently stir the meatballs to coat with sauce. Serve warm over rice or noodles.