Cut the onions into thick chunks and cut the scallion greens into one-inch pieces.
Place the cubed pork belly in a dry pan over medium heat and brown for a few minutes, letting the fat render out, then remove from the heat and set aside.
Add the onions and minced garlic to the rendered fat and cook until fragrant.
Return the pork to the pan along with the dark soy sauce, light soy sauce, oyster sauce, sugar, sesame oil, black pepper, and water. Bring to a simmer.
Stir the cornstarch and cold water together and add the slurry along with the scallion greens. Remove from the heat once the greens are wilted and the sauce has thickened.
Serve over a fresh bowl of rice. Optionally garnish with more black pepper. But it's your food, you eat it how you like.