Preheat oven to 375°F/190°C. Butter an 8 x 8-inch 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.

Slowly whisk in the milk until thickened. Stir in the fontina and half of the parmesan cheese until melted and smooth. Stir in the sage, nutmeg, and pepper. Taste for salt.

Arrange ⅓ of the butternut squash slices in a single layer in the dish. Pour ⅓ of the sauce over the squash. Repeat with another layer of squash and sauce, then top with the remaining squash and sauce.

Mix the rest of the parmesan cheese with the panko and top the gratin with an even layer. Add more sage if desired.

Place the dishes onto a baking sheet, cover with foil, and bake for 30 minutes. Remove the foil and continue to bake until the top is golden and the butternut squash is tender.

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