In a large skillet (with a lid), combine the brown sugar, vinegar, ketchup, soy sauce and 2 cups of the pineapple juice. Stir the mixture around and bring it to a gentle boil over medium-high heat. To thicken the sauce, make a slurry by mixing the cornstarch with the remaining 1/4 cup pineapple juice in a small bowl until smooth. Add it to the sauce, whisking to combine.
Add the frozen meatballs, sriracha and bell pepper and toss to combine. Cover the skillet and cook until the sauce has thickened and the meatballs are heated through, 8 to 10 minutes.
Stir in the pineapple, then sprinkle in 2 tablespoons of the green onions. Serve the meatballs and sauce over rice and sprinkle on the rest of the green onions at the end.
Combine the beef, breadcrumbs, milk, parsley, cream, mustard, salt, black pepper, red pepper flakes and eggs in a large bowl. Knead it all together with your hands until well combined.
Scoop out 1-tablespoon portions of the meat mixture and roll into neat balls. Put them on parchment paper-lined rimmed baking sheets as you go, then put the sheets in the freezer for about 10 minutes to firm them up before frying.
To brown the meatballs, heat 1/4 cup olive oil in a large skillet over medium-high heat. Working in batches, add some meatballs to the skillet and cook on all sides until they have great color on the surface and are fully cooked inside, 5 to 6 minutes. Drain the meatballs on paper towels when they're done, then line them up on clean parchment paper-lined baking sheets. Cook the remaining meatballs.
Put the baking sheets in the freezer, uncovered, until the meatballs are frozen and firm on the surface, 30 to 45 minutes. Then just pop the meatballs into 5 to 7 freezer bags (roughly 25 per bag) and freeze them immediately. They'll be there when you need them.