1 cup heavy cream
1 cup milk
1 teaspoon salt
1/4 teaspoon freshly ground black pepper
1/4 teaspoon freshly grated nutmeg
1 pound celery root (also known as celeriac), peeled and sliced (thick slice on a mandolin)
1 pound boiling potatoes (such as Red Bliss, Yukon Gold), thinly sliced
3 tablespoons unsalted butter, plus some for buttering pan
1 teaspoon fresh thyme sprigs, or chopped parsley