Kohlrabi and Potato Gratin

2 Tbs. butter 
2 cups heavy cream
2 tablespoons crème fraîche or sour cream
1 cloves garlic, minced
1/2 teaspoon minced thyme
Salt and freshly ground pepper
4 large potatoes (2 pounds), peeled
1 large kohlrabi, peeled

Preheat the oven to 300°. Butter a 9-by-13-inch glass baking dish. In a skillet, melt the butter and add the thyme and garlic and let simmer a few minutes to release the flavors.  Add the heavy cream and sour cream or crème fraîche and plenty of salt and pepper.

Using a vegetable mandolin (or a knife and a skilled hand), thinly slice the potatoes and kohlrabi, then add to the cream and toss to mix. Spread the potatoes, kohlrabi and cream in an even layer in the prepared baking dish and bake for 15 minutes.

Increase the oven temperature to 325° and bake for about 1 hour and 15 minutes longer, or until the potatoes are tender and the top is browned. Let the gratin stand for 10 minutes before serving.