In a large mixing bowl, combine ground beef, breadcrumbs, diced onion, egg, salt, and pepper. Mix thoroughly but gently until just combined - overmixing can make the meatballs tough.

Using clean hands, roll the meat mixture into 1-inch balls. You should get approximately 20-24 meatballs from 1 pound of beef.

Heat olive oil in a large skillet over medium heat. Add the meatballs in batches, being careful not to overcrowd the pan. Brown on all sides, about 2-3 minutes per batch. They don't need to be fully cooked through at this point.

In a medium saucepan, combine brown sugar, apple cider vinegar, ketchup, soy sauce, and minced garlic. Bring to a simmer over medium heat, stirring occasionally until sugar dissolves completely.

Whisk the cornstarch slurry into the simmering sauce. Continue to cook for 1-2 minutes until the sauce thickens to a glossy consistency that coats the back of a spoon.

Return all the browned meatballs to the skillet. Pour the thickened sauce over them. If using bell peppers and pineapple, add them now. Gently stir to coat everything in sauce.

Reduce heat to low, cover, and simmer for 15-20 minutes until meatballs are cooked through and flavors have melded. Stir occasionally to ensure even coating.

Serve hot over white rice, garnished with sliced green onions and a sprinkle of sesame seeds if desired.