Potato and Celeriac Gratin

Name: Potato and Celeriac Gratin
Ingredients: 1½ pounds large Yukon Gold or russet potatoes, peeled and very thinly sliced; 1½ pounds celeriac (celery root), peeled and very thinly sliced; 2 cups whole milk; 2 cups cold water; 3 plump cloves garlic, peeled, halved, green germs removed, minced; ¾ teaspoon fine sea salt; 3 bay leaves, preferably fresh; A bunch of fresh thyme, tied with cotton twine or secured in a wire mesh ball; Freshly grated nutmeg; Freshly ground black pepper; 1 cup heavy cream; 2 cups (about 8 ounces) freshly grated imported French or Swiss Gruyere cheese; 1 tablespoon fresh thyme leaves; A 2-quart gratin dish