In a large bowl, combine ground beef, ground pork, breadcrumbs, minced onion, garlic, egg, soy sauce, ginger, salt, and pepper. Using clean hands, gently mix until just combined. Avoid over-mixing to keep meatballs tender.
With slightly damp hands, form the mixture into uniform 1-inch balls. You should yield approximately 24-30 meatballs. For consistent sizes, use a cookie scoop or tablespoon measure. Place the formed meatballs on a parchment-lined tray.
Heat 2 tablespoons of neutral oil in a large skillet over medium-high heat. Working in batches to avoid crowding, sear the meatballs on all sides until golden brown, about 2 minutes per side. They don't need to be fully cooked through at this point. Transfer the browned meatballs to a plate.
In a medium saucepan, combine pineapple juice, brown sugar, rice vinegar, ketchup, and soy sauce. Bring to a gentle simmer over medium heat. In a small bowl, whisk together cornstarch and water to create a slurry, then slowly pour into the simmering sauce while whisking constantly. Continue to cook until the sauce thickens enough to coat the back of a spoon, approximately 3-5 minutes.
Return the meatballs to the skillet or transfer to a slow cooker. Add the bell peppers and pineapple chunks, then pour the sweet and sour sauce over everything. If cooking on the stovetop, simmer covered for 15-20 minutes until meatballs are cooked through. If using a slow cooker, cook on low for 3-4 hours.
Serve hot over steamed rice for a main course, or with toothpicks as an appetizer.