1. Preheat the oven to 400°F (200°C).

2. Slice the squash into thin layers and arrange them in a single layer on a baking sheet lined with parchment paper. Drizzle with 1 tbsp of olive oil and roast in the oven for about 10 minutes or until the squash has slightly softened and released its moisture.

3. Reduce the oven temperature to 350°F (175°C).

4. Peel and finely chop the onion and garlic. Heat the remaining 1 tbsp of olive oil in a skillet over medium heat. Sauté the onion and garlic until softened and translucent about 5 minutes. Add the crushed tomatoes salt pepper and chopped basil. Simmer for 15 minutes stirring occasionally.

5. In a separate bowl whisk together the eggs and 3.4 fl oz (1 dl) of the tomato sauce.

6. In a greased 12x8 inch (30x20 cm) oven-safe dish create a layer of tomato sauce. Arrange a third of the roasted squash slices on top followed by a third of the Parmesan cheese. Repeat this process alternating between the tomato sauce squash Parmesan cheese and mozzarella cheese finishing with a layer of tomato sauce on top.

7. Finally pour the egg and tomato sauce mixture evenly over the top layer ensuring all the ingredients are well coated. Sprinkle with the remaining Parmesan cheese and bake in the preheated oven for 25-30 minutes or until the gratin is golden brown and bubbly.

8. Serve warm garnished with fresh basil leaves and accompanied by a side salad and crusty bread.