Combine all meatball ingredients in a large bowl and use your hands to mix until evenly combined. Form into about 1 tablespoon size meatballs, about 20 meatballs.
Heat a large nonstick pan over medium-high heat. When hot, add the meatballs (they will release fat as they cook so no need to add more). Cook until golden, turning occasionally until browned on all sides, about 10-15 minutes. Cook in batches if you need to. Transfer to a plate and set aside, keeping as much fat in the pan as possible. Do not clean the pan.
While meatballs cook, stir together all sauce ingredients and set aside.
Add the onions and bell pepper to the pan and cook on high, until the peppers are lightly charred and the onions and peppers are tender crisp, about 5 minutes. Transfer to the plate with the meatballs and set aside. Do not clean the pan.
Add the sauce and cook until thickened and glossy, about 5 minutes. Add back the meatballs and vegetables and stir in the pineapple. Toss to coat everything evenly. Sprinkle with greens onions and sesame seeds, if using and serve.