Combine the sugar, salt, and 2 tablespoons cornstarch in a medium saucepan. Add the orange juice, pineapple juice, vinegar, and tomato paste and stir until blended. Cook over medium heat, stirring constantly, until sauce is thickened. Reduce heat to low.

Place 2 tablespoons of oil in a hot wok and add the garlic and hot pepper flakes. Stir fry for a few seconds, then add the bell peppers, pineapple, and onions. Stir fry until vegetables are hot through, but still slightly crisp.

Arrange cooked vegetables and pork cubes with steamed rice on a serving plate and serve with the sauce on the side.