Preheat the broiler to high with the rack about 4 inches from the heating element; line a sheet pan with foil and grease it generously with cooking spray.
Prepare produce: grate the outer layer of the yellow onion to measure 1/2 cup; grate garlic and fresh ginger on a fine grater to yield 1 teaspoon each; cut bell peppers into 1-inch pieces.
Drain the pineapple, reserving 3 tablespoons of the juice, and measure 1/2 cup mandarin oranges including their juice.
Make meatballs: in a mixing bowl combine 2 lb ground beef, 2/3 cup panko, 2 tbsp dried minced onion, 1 large egg, 1/4 cup coconut milk, 1-1/2 tsp ground ginger, 2 tsp salt, and 1/4 tsp pepper; mix gently until just combined.
Lightly oil your hands or spray them, then form the mixture into meatballs about 1½ tablespoons each (approximately 30–32 meatballs). Place them on the prepared sheet pan and drizzle 1 tablespoon oil over them.
Broil the meatballs, flipping every 2 minutes, until an instant-read thermometer registers 160°F; total broiling time will be about 4–8 minutes—start checking at 4 minutes.
While meatballs cook, heat 1 tablespoon oil in a large pan or wok over medium heat; add the grated onion, grated ginger, and grated garlic and cook, stirring, for 2–3 minutes.
Add the chopped bell peppers and cook, stirring occasionally, 4–6 minutes until mostly crisp-tender.
Make the sauce: to the pan with veggies add 1/3 cup sugar, 1/4 cup rice vinegar, 3 tbsp reserved pineapple juice, 3 tbsp ketchup, 1/2 tsp Worcestershire sauce, 1 tbsp soy sauce, 1 tsp oyster sauce, and the 1/2 cup mandarin oranges with their juices; lightly break the oranges with the spoon.
In a small bowl whisk 1 teaspoon cornstarch with 1/2 cup (109 g) water until smooth, then pour the slurry into the pan while whisking; bring to a rapid simmer and cook 1–2 minutes, then reduce heat and simmer another 1–2 minutes until thickened.
Stir in the drained pineapple chunks and 1/8 teaspoon sesame oil; remove the pan from heat.
To serve, divide cooked rice among plates, top with broiled meatballs, and spoon the sauce and vegetables over the meatballs; garnish with toasted sesame seeds and thinly sliced green onion if desired.