PREP: Heat broiler to high broil with the rack 4 inches from the heating element. Line a sheet pan with foil and generously grease with nonstick cooking spray.
PREP VEGGIES/FRUIT: Start by prepping the veggies. Peel the outer layer of the onion and grate on a box grater to get 1/2 cup grated onion. Peel garlic and ginger on a microplane to get 1 teaspoon of each. Cut bell peppers into 1-inch pieces. Measure out 1/2 cup mandarin oranges (spoon both the oranges and juices into a 1/2 cup measuring cup). Drain out 1 cup of pineapple chunks, reserving 3 tablespoons of the juice for the sauce.
MEATBALLS: Combine the meatball ingredients, seasoning to taste with salt and pepper (I use 2 teaspoons fine sea salt and 1/4 teaspoon pepper). Mix with your hands until just combined; rub a little oil on your hands (or spritz with cooking spray) and then roll the meat mixture into meatballs (1 and 1/2 tablespoons in size; about 30-32 total meatballs). Place rolled meatballs on the prepared sheet pan and drizzle with 2 tablespoons oil. Place in the oven and broil, flipping meatballs over, every 2 minutes until cooked through (until a meat thermometer reads 160 degrees F). This takes anywhere from 4-8 minutes. Start checking the temperature at 4 minutes (in my oven this takes about 5-6 mins).
VEGGIES: Meanwhile, add the 1 tablespoon oil to a large skillet or wok over medium heat. Once hot, add in the grated onion, ginger, and garlic. Cook, stirring frequently, for 2-3 minutes. Add in the chopped bell peppers and stir around for 4-6 minutes or until mostly crisp tender.
SAUCE: To the skillet, add the white sugar, rice vinegar, pineapple juice, ketchup, Worcestershire sauce, soy sauce, oyster sauce, and mandarin oranges with juices. Mix to combine and slightly smash down the oranges. In a separate bowl, whisk together the cornstarch and water until smooth. Pour the cornstarch slurry into the sauce while whisking constantly. Bring to a rapid simmer and simmer for 1-2 minutes, and then reduce the heat to medium-low and continue to simmer for another 1-2 minutes or until sufficiently thickened. Stir through the pineapple chunks. Remove from heat and stir in the sesame oil.
SERVE: Place meatballs over cooked white rice (See Note 3). Generously spoon sauce and veggies over the meatballs. Garnish with sesame seeds and green onions if desired. Enjoy immediately.
STORAGE: If there are leftovers, I recommend storing the sauce, meatballs, and rice all separately. Each will stay good for 4-5 days in an airtight container in the fridge.