To make soy-garlic glaze, combine onion, garlic, soy sauce, mirin, brown sugar, garlic powder and ginger in a saucepan. Place over low-medium heat and stir until sugar is fully dissolved. In a small bowl, place cornstarch and a tablespoon of the soy-garlic mixture. Mix to dissolve. Add to the saucepan and stir until slightly thickened.