Mix the oyster sauce, soy sauce, sugar, and water. Set aside.
Heat oil in a wok or large pan over medium-high heat. Fry the pork belly until browned and crisp. Transfer to a plate. If using ground pork, cook until browned.
Remove the extra oil, leaving about 1 tablespoon in the pan. Lower the heat to medium and cook the garlic and onion for about 2 minutes, until softened.
Add the tomatoes and cook for about 5 minutes, until soft and starting to break down. Add the peppers (if using) and cook for a minute.
Add the chayote and prepared sauce. Cook for 3 to 5 minutes, until the chayote is tender but still slightly crisp. Season with salt and pepper to taste.
Return the pork to the pan and mix well. Turn off the heat.
Notes
Chayote: Upo, zucchini, or summer squash will also work.
Pork: Boiled pork belly is easier to slice and gets crisp when fried (like lechon kawali). Ground pork, chicken, shrimp, or tofu will also work.
Peppers: Optional. Use long green or Korean peppers for mild heat, or bell peppers for no heat.