Cheesy Cabbage Gratin

Name: Cheesy Cabbage Gratin
Ingredients: 1 (2¼–2½ pound) head Savoy or green cabbage , outermost leaves removed; 4 tablespoons extra-virgin olive oil , divided; ¼ teaspoon ground pepper; ¼ teaspoon salt; ½ cup lower-sodium no-chicken or vegetable broth; 1½ teaspoons cornstarch; 1 shallot, thinly sliced lengthwise; 2 cloves garlic , thinly sliced; 2 teaspoons fresh thyme leaves , plus more for garnish; 1 teaspoon chopped fresh rosemary; 1 teaspoon crushed red pepper (optional); 1 cup heavy cream; 1½ cups finely grated Parmesan cheese , divided; ½ cup shredded Gruyère cheese