Sweet Potato Gratin

Name: Sweet Potato Gratin
Ingredients: 1 large sweet potato, cleaned, peeled and sliced very thinly; 1 large Russet potato, cleaned, peeled and sliced very thinly; 2 cups half and half or whipping cream; 1 teaspoon fresh minced thyme; 1/2 teaspoon fresh minced tarragon; 1 teaspoon vanilla extract; 1 pinch freshly grated nutmeg; 1 cup shredded Parmigiano Reggiano; unsalted butter, as needed; salt and white pepper, to taste