In a large mixing bowl, combine the ground beef, breadcrumbs, milk, egg, crushed garlic, finely chopped onions, Worcestershire sauce, and rosemary if using. Mix thoroughly until all ingredients are well incorporated.

Shape the mixture into meatballs—smaller for appetizers or larger for a main course. Arrange the meatballs evenly on a baking sheet lined with parchment paper or lightly greased. Bake in a preheated oven at 350°F (175°C) for about 15–20 minutes, or until browned and cooked through, depending on their size.

Meanwhile, prepare the sauce in a large skillet. Whisk together the brown sugar and cornstarch until evenly blended. Stir in the entire can of pineapple tidbits along with the syrup, then add vinegar and soy sauce. Bring the mixture to a gentle boil over medium heat, stirring constantly until it thickens slightly.

Reduce the heat to low and gently add the cooked meatballs to the sauce. Cover and let them simmer for about 10 minutes, allowing the flavors to meld.

Toss in the chopped green pepper and continue to simmer uncovered for an additional 5 minutes, just until the pepper softens slightly but still retains a bit of crunch.

Serve hot as an appetizer or over rice for a satisfying main dish.