Combine water, brown sugar, garlic, and onion powder in a small saucepan and cook over medium heat until sugar dissolves. Add pineapple juice and ground ginger and simmer for 10 minutes.

In a small bowl, mix cornstarch with ¼ cup cold water.

Stir into saucepan and continue to simmer until sauce reaches your desired thickness. Add a bit more water if sauce gets too thick for your liking.