Heat the oil in a large pot over medium-high heat. Cook the onion and garlic in the hot oil until the onions are tender. Stir in the tomato; cook until soft, about 3 minutes. Crumble the ground beef into the pan and cook until brown, about 5 minutes. Add the water, potato, beef bouillon, fish sauce, salt, and pepper; reduce heat, cover, and simmer until the potatoes are tender, stirring occasionally, about 30 minutes.