Preheat the oven to 500˚. Combine the squash, rosemary, olive oil, 1 teaspoon salt and a few grinds of pepper on a rimmed baking sheet. Toss to coat, then spread the squash in a single layer. Roast until just tender but not completely cooked through (the squash will finish cooking on the pizza), about 10 minutes. Remove the squash to a plate; discard the rosemary.

Let the baking sheet cool for 5 minutes, then stretch the pizza dough on the pan with your fingers until it reaches the edges. Sprinkle with the mozzarella and pecorino.

Combine the ricotta with the remaining ½ teaspoon salt and a few grinds of pepper in a medium bowl.

Dollop the ricotta all over the pizza. Scatter the butternut squash on top of the cheese, making sure that some of the ricotta is showing through. Gather the prosciutto into clusters and scatter over the pizza.

Bake until the cheese is melted and starting to turn brown in spots and the dough is completely cooked through and golden brown around the edges, 20 to 25 minutes. Let cool for 5 minutes before slicing.