One 2-pound butternut squash—peeled, seeded and sliced 1 inch thick (I used a 2 lb container from Costco)
4 tbs. olive oil, plus more for the pan
1 large onion, finely chopped
1/2 teaspoon dried sage, crumbled
Freshly ground pepper and salt
6 cups water
2 cups coarse polenta
4 ounces mozzarella, shredded (1 cup)
1/4 cup plus 2 tablespoons freshly grated Parmigiano-Reggiano cheese
Preheat the oven to 375°. Arrange the squash on a large oiled baking sheet and roast for 30 minutes, until tender. Transfer the squash to a bowl and mash coarsely. (You should have about 1 1/2 cups.)
Meanwhile, in a skillet, heat the 4 tbs of olive oil. Add the onion and sage and season with pepper. Cover and cook over moderate heat, stirring, until golden, 25 minutes; add a few tablespoons of water if the onion looks dry.
In a saucepan, bring the 6 cups of water to a boil. Whisk in the polenta and 1 teaspoon of salt. Cook over very low heat, stirring frequently, until tender, about 35 minutes. Stir in 1 tablespoon of butter along with the mashed squash, onion, mozzarella and half of the grated Parmigiano cheese. Pour the hot polenta into a buttered casserole dish. Flatten the top of the polenta slightly without spreading it to the edges; refrigerate until firm, about 3 hours.
Rub the surface of the polenta with softened butter and sprinkle with the remaining grated Parmigiano cheese. Bake in a 375° oven for 1 hour, until the top and side are lightly browned and crisp. Let stand for 20 minutes before cutting it into wedges.
Serve it with fresh or braised greens and tomato sauce.