Layers of pumpkin, butternut squash, and potatoes are baked in a rich, cheesy sauce until meltingly tender. Serve as a side dish with roasted chicken or pork, or as a vegetarian main dish alongside a green salad.
Heat oil in a medium saucepan over medium. Add flour, nutmeg, and cloves to pan. Cook, stirring constantly, 1 minute. Add milk; bring to a boil, and cook, stirring often, until thickened, about 1 minute. Remove from heat, and let stand 5 minutes. Add 3/4 cup of the cheese, 1/4 cup at a time, stirring with a whisk until melted after each addition. Whisk in 1 1/2 teaspoons of the salt.
Preheat oven to 425°F with an oven rack about 8 inches from heat.
Combine bread, parsley, and thyme in a food processor; pulse until coarsely chopped. Transfer to a large bowl, and toss with melted butter. Set aside.
Place potato, squash, and pumpkin in a lightly greased 13- x 9-inch baking dish. Pour milk mixture over squash mixture, and sprinkle with remaining 1/2 teaspoon salt, pressing mixture into an even layer. Cover and bake in middle of preheated oven until squash is tender, about 1 hour. Remove baking dish.
Preheat broiler to HIGH. Uncover dish, and sprinkle top of gratin with bread-herb mixture and remaining1/2 cup cheese. Broil on rack 8 inches from heat until top is browned, 2 to 3 minutes.