Heat oven to 400° F.
Cut the squash into 1/2-inch-thick slices, then cut each slice into 1-inch chunks. Place the squash and onion on a baking sheet. Drizzle with the oil, season with the salt and pepper, and toss. Roast until tender, about 20 minutes. Transfer to a plate.
Increase oven temperature to 450° F. Roll the dough out 1/4 inch thick. Clean the baking sheet and sprinkle it with the cornmeal. Place the dough on top. Scatter the squash and onion mixture over the dough, sprinkle with the thyme, and add dollops of the ricotta. Bake until golden, about 25 minutes. Slice into wedges.