Steps
1. Preheat the oven to 375°F (190°C).
2. In a small bowl, whisk together the eggs and mustard. Set aside.
3. Heat a tablespoon of oil in a non-stick ovenproof skillet over medium heat.
4. Add the diced onions and sauté until translucent.
5. Add the sliced potatoes and cook until they are tender and lightly browned.
6. Pour the egg and mustard mixture over the potatoes and onions in the skillet.
7. Arrange the tomato slices on top of the eggs.
8. Season with salt and pepper.
9. Transfer the skillet to the preheated oven and bake for about 15-20 minutes, or until the eggs are set.
10. Remove from the oven and let it cool slightly before slicing.
11. Serve warm and enjoy!