Prepare the christophines: Peel the christophines under cold water, cut them in half and remove the central core. Cook them in boiling salted water for 20 minutes until tender. Drain them and let them cool.

Core and puree: Once cooled, peel the christophines. Mash the flesh in a bowl using a fork to obtain a puree.

Sauté the onion and garlic: Heat the olive oil in a skillet over medium heat. Add the finely chopped onion and crushed garlic, then fry until translucent. Add the Christophine puree to the pan.

Prepare the gluten-free béchamel: In a small saucepan, heat 200 ml of milk and 15 g of butter over low heat. In a bowl, dilute 20 g of cornstarch in a little cold water. Add this mixture to the hot milk while stirring constantly to avoid lumps. Continue stirring until the sauce thickens. Season with salt, pepper, a pinch of nutmeg, and thyme.

Mix the béchamel with the Christophine puree: Pour the béchamel over the Christophine puree and mix well. Let it simmer for a few minutes so that the flavors combine.

Assemble the gratin: Preheat the oven to 180°C (350°F). Fill a gratin mold with the mixture. Sprinkle with grated cheese and, if desired, add gluten-free breadcrumbs for a crunchy effect.

Bake: Bake for 20 minutes, or until the top is golden brown. If necessary, place them under the grill for a few minutes to obtain an even golden crust.

Serve: Remove the gratin from the oven and serve it, as a side dish or as a main dish.