In a large bowl, stir breadcrumbs and milk together. Let sit 2 minutes.
Add egg, onion, garlic, salt, pepper, and optional ginger. Stir to combine.
Add ground beef (and pork if using). Mix gently just until combined.
Roll into 1 ½-inch meatballs (about 2 tablespoons each).
Heat a large skillet or Dutch oven over medium-high heat. Add a drizzle of oil. Brown meatballs in batches, turning to brown most sides (not cooked through). Transfer to a plate.
Reduce heat to medium. Whisk pineapple juice, ketchup, vinegar, brown sugar, soy sauce, garlic, and ginger (optional) in the pan. Simmer 2–3 minutes.
Whisk cornstarch and cold water until smooth. Stir into the sauce and simmer 1–2 minutes until glossy.
Return meatballs to the pan. Cover, reduce heat to low, and simmer 12–15 minutes, until meatballs reach 160°F internally.
If using bell peppers, add them for the last 8–10 minutes. If using pineapple chunks, add them for the last 2–3 minutes.
Serve hot over rice or noodles, or keep warm in a slow cooker for parties.