Add the garlic and onion to the roux, and continue to cook for another minute, or until the roux is fragrant and golden.
Stir in the oyster sauce, soy sauce, sesame oil, and a pinch of white pepper. Bring the mixture to a simmer.
Continue to cook until the gravy thickens enough to coat the back of a spoon, about 3 to 5 minutes. Taste and adjust the white pepper if needed.