Heat oil in a pan. Add fish, and fry lightly until just brown. Transfer fish to a clay pot or stove-top casserole. Brown pork in pan and add to fish along with garlic, stock, sugar and chili. Stir to combine. Cover and cook on medium heat until sauce is thickened.
In pan, sauté onion slices until tender. Add fish sauce to pot and cook, stirring. When cooked, place onions on top of pork and fish. Place fresh tomato on top just before serving.