Potato/Butternut Squash Gratin

Position a rack in the center of the oven and heat the oven to 400 degrees F. Butter or oil a 3-quart gratin dish; set aside.

Put the potatoes, butternut squash, milk, 1/2 tsp. salt, and a few grinds of pepper in a 12-inch skillet. Simmer, partially covered, over medium to medium-low heat, stirring occasionally and gently with a rubber spatula until the potatoes are barely tender when pierced with a fork or skewer, 8 to 12 minutes.

In a medium skillet, heat butter or oil over medium-high heat and saute onions until tender, fragrant, and lightly browned. Season to taste with salt and pepper.

Using a slotted spoon, transfer half the potato mixture to the prepared gratin dish, spreading it evenly. Layer on the onions, prosciutto, Fontina, and chopped fresh thyme. Top with the remaining potato mixture spreading it evenly, and pour over any liquid remaining in the pan.

Evenly scatter the topping mixture over the potatoes. Bake the gratin until it's bubbly, the top is brown, and the potatoes are completely tender when poked with a fork or a skewer, 25 to 30 minutes. Let the gratin sit for at least 10 and up to 30 minutes before serving so the liquid is fully absorbed and the layers are cohesive.