Preheat the oven to 180 ° C fan oven. Clean, peel and thinly slice the pumpkin. Wash and dice the tomatoes. Peel the garlic. Wash and clean the leek, cut into thick rings. Wash the parsley, shake dry and finely chop. Cut the cheese into small pieces.
Heat the oil in a pan and fry the pumpkin over a medium heat while stirring for about 5 minutes. Season to taste with salt and pepper and remove. Add the tomatoes and leek to the pan. Add the garlic through the press. Sweat everything for about 2 minutes.
Season to taste with chilli sauce. Mix in the parsley and season with salt and pepper. Spread the pumpkin in an ovenproof dish. Pour the tomato-leek mixture over it and sprinkle with the cheese. Bake the pumpkin in the oven for at least 20 minutes. If the cheese gets too dark, cover with aluminum foil.
The gratin tastes hot, but also very good when it is lukewarm.