Pumpkin Gruyère Gratin with Thyme

Preheat oven to 425 °F. In a medium skillet over medium heat, heat the oil and butter. Add the onion and 1/2 teaspoon salt and cook until softened and starting to brown, 8 to 10 minutes, tossing occasionally. Transfer to a 3-quart gratin dish and add pumpkin, thyme leaves, and 1/2 teaspoon salt, and pepper and toss to combine. Roast until squash is soft, tossing halfway through, about 45 minutes. Meanwhile, in a small saucepan, combine cream, reserved thyme stems, garlic, remaining 1/2 teaspoon salt, cayenne, and nutmeg. Bring to a simmer, whisk in cornstarch, and cook for 1 minute. Turn off heat, cover, and let steep about 30 minutes. Once squash is tender, remove thyme stems from cream mixture and pour cream over squash. Sprinkle with cheese and then breadcrumbs. Return to oven and cook until cream is bubbling, 5 to 7 minutes. Broil until top is golden brown, 2 to 3 minutes. Serve immediately.