1. Preheat your oven to 375°F (190°C). Heat olive oil in a large oven-safe skillet over medium heat. Sauté potatoes until golden and tender, about 10 minutes. Remove and set aside.
2. In the same skillet, cook ground beef and onion until browned. Drain excess grease. Stir in taco seasoning and water. Simmer for 3-4 minutes until thickened.
3. Add cooked potatoes back into the skillet, tossing to combine with the beef mixture.
4. Sprinkle cheese evenly over the top and transfer to the oven. Bake for 10 minutes, or until the cheese is melted and bubbly.
5. Remove from the oven and top with diced tomato and green onions. Serve hot with tortilla chips or sour cream on the side.