1. 1. Preheat the oven to 180 degrees Celsius.

2. 2. Wash and cut the vegetables into small pieces.

3. 3. Bring a pot of water to a boil and blanch the vegetables until they are al dente.

4. 4. Drain the blanched vegetables and set aside.

5. 5. In a pan, sauté the leeks in some oil until they are soft.

6. 6. Add the fresh tomatoes and cook for another 2 minutes.

7. 7. Add the blanched vegetables to the leeks and tomatoes and mix well.

8. 8. Grate the cheddar cheese.

9. 9. Grease a baking dish and place the vegetables in it.

10. 10. Sprinkle the grated cheddar cheese over the vegetables.

11. 11. Place the baking dish in the preheated oven and bake for about 20-25 minutes, until the cheese is golden brown and crispy.

12. 12. Remove the vegetable gratin from the oven and let it cool slightly before serving.