Preheat oven to 375°F/190°C.

Butter an 8 x 8-inch (20 x 20-cm) baking dish or mini cast iron skillets.

Melt the 4 tablespoons butter in a pan over medium heat. Whisk in the flour until it absorbs the butter. Cook while stirring for 2 minutes to cook out the flour taste, it will start to turn darker when ready. Slowly whisk in the milk until thickened. Stir in the fontina and half of the Parmesan cheese until completely melted and sauce is smooth. Stir in the sage, nutmeg and pepper. Taste the sauce and taste for salt. This will depend on how salty your cheese is.

Arrange ⅓ of the butternut squash slices in a single layer, slightly overlapping on the bottom of the dish.

Pour ⅓ of the sauce over the first layer of butternut squash. Arrange another single layer of butternut squash and top with ⅓ of the sauce. Arrange the last layer of butternut squash and top with the remaining sauce.

Mix the rest of the Parmesan cheese with the panko and mix well. Top the gratin with an even layer of the panko and a little more sage if desired.

Place the dishes onto a baking sheet, cover with foil and bake for 30 minutes. Time will depend on how thickly you sliced the squash. Remove the foil and continue to bake until the top is golden and the butternut squash is tender when a fork is inserted. 

Remove from the oven and allow to sit for 5 minutes before serving.