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

Hollow out and purée: Once cooled, peel the chayotes. Mash the flesh in a bowl with a fork to obtain a purée.

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

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

Mix the béchamel sauce with the chayote puree: Pour the béchamel sauce over the chayote puree and mix well. Let it simmer for a few minutes to allow the flavors to blend.

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

Bake: Bake for 20 minutes, or until the top is golden brown. If needed, broil for a few minutes to get an even more golden crust.

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