Potato and Parsnip Gratin
3 Yukon Gold potatoes, peeled
2 parsnips, peeled
2 cloves garlic, minced
1 tablespoon butter, melted
salt and ground black pepper to taste
1 teaspoon fresh thyme leaves, divided
2 ounces finely grated Parmigiano-Reggiano cheese, divided
¾ cup creme fraiche, divided
1 cup chicken broth
1 pinch cayenne pepper, or to taste
Place potatoes and parsnips in a bowl of cold water.
Preheat oven to 375 degrees F (190 degrees C). Place garlic and melted butter into a 2-quart baking dish; brush garlic and butter all over the inside of the dish.
Slice potatoes very thin using a mandolin slicer.
Layer 1/3 potato slices into the bottom of the prepared baking dish. Season with salt and black pepper. Sprinkle a few thyme leaves onto potato slices and a light dusting of Parmigiano-Reggiano cheese. Spoon about 3 tablespoons creme fraiche over cheese.
Thinly slice parsnips using a vegetable peeler; arrange 1/2 the parsnip slices in an even layer over the creme fraiche and season with salt and black pepper. Repeat potato and parsnip layering process: 1/3 potato slices, salt and black pepper, thyme, Parmigiano-Reggiano cheese, creme fraiche, and remaining 1/2 parsnip slices. Dust with salt. Arrange last 1/3 potato slices on top and season with salt.
Pour chicken broth over potato-parsnip layers about 3 tablespoons at a time. Shake dish gently to eliminate air bubbles. Gently spread remaining 2 tablespoons creme fraiche over the potatoes. Sprinkle cayenne and remaining Parmigiano-Reggiano cheese over the top.
Bake in the center of the preheated oven until gratin is browned and a knife pierces layers easily, 45 minutes to 1 hour.