Pumpkin Gruyère Gratin with Thyme

Name: Pumpkin Gruyère Gratin with Thyme
Ingredients: 1 teaspoon extra-virgin olive oil; 1 teaspoon butter; 1 medium onion, halved and sliced; One 3 1/2-pound sugar pumpkin, kabocha, or butternut squash, peeled, seeded, and cubed (about 9 cups); 1 tablespoon fresh thyme leaves, stems reserved; 1 1/2 teaspoons kosher salt, divided; 1/2 teaspoon freshly ground pepper; 3/4 cup heavy cream; 2 garlic cloves, very thinly sliced; Pinch cayenne pepper; Freshly grated nutmeg; 2 teaspoons cornstarch; 3 1/2 ounces Gruyère cheese, grated (1 heaping cup); 1/3 cup Japanese-style breadcrumbs (panko)