In a large bowl, combine beef, panko, egg, 2 cloves minced garlic, salt, and pepper until evenly combined. Scoop mixture into 1-inch balls and place on a clean plate.
Heat a large skillet over medium-high heat. Once pan is hot, add vegetable oil and swirl pan to evenly coat. Add meatballs to pan and sear on all sides, turning occasionally with a tongs until meatballs are golden brown. Reduce heat to medium and continue cooking meatballs, turning occasionally until meatballs register 165F. Transfer meatballs to a clean plate and set aside
Drain all but about 1 tablespoon drippings from pan and set back over medium-high heat. Add peppers to pan and sauté over medium-high heat until peppers are crisp-tender, about 2-4 minutes. Add remaining mincedgarlic and green onion and cook 1-2 additional minutes until fragrant.
Add meatballs, pineapple, and sweet and sour sauce to peppers and cook 3-5 minutes over medium-high heat until sauce is slightly thickened and coating meatballs and peppers. Season sauce with salt and pepper to taste if needed.
Serve meatballs and peppers warm with steamed rice and additional green onions on top. Enjoy!