In a large pan or wok, heat the cooking oil over medium heat.
Add the minced garlic and sliced onion. Sauté until fragrant and the onions are soft. Then, add the chopped tomato and cook until it becomes soft and releases its juices.
Add the sliced pork belly to the pan. Stir-fry for about 5-7 minutes, or until the pork turns golden brown and starts to render its fat.
Pour in the soy sauce and oyster sauce. Stir well to coat the pork with the sauces.
Pour in 1/2 cup of water. Bring the mixture to a simmer, then lower the heat and let it cook for another 5 minutes to tenderize the pork.
Add the sliced red bell pepper, cabbage, and green onions to the pan. Stir-fry for another 3-5 minutes, or until the vegetables are cooked but still slightly crisp.
Season with salt and pepper to taste. Adjust the seasoning as needed.
Serve hot with steamed rice and enjoy!