Potato Gratin with Horseradish and Parmesan

    adapted from Shaw McClain's Yukon Gold Potato Gratin Recipe for Epicurious 10/06

1/2 bunch fresh thyme
1/2 tbsp fresh black peppercorns
1 bay leaf
2 garlic cloves, peeled and smashed with back of knife

1 1/2 pounds potatoes peeled and cut crosswise into 1/8 inch slices
1/2 tablespoon sea salt
2 cups heavy cream

1/8 cup prepared horseradish 
1 cup Parmesan cheese, coarsely grated  (we used a mixture of Parmesan and sharp raw white Cheddar)

Place a rack in middle position and preheat oven to 400 degrees.

Generously butter a 1 1/2 gratin dish (we used a deep dish pie plate).

Make a bouquet garni with first four ingredients.

In heavy 3 saucepan combine potatoes, bouquet garni, salt and cream.  Set over moderate heat and bring to a boil.  Reduce heat to low and simmer uncovered until potatoes can just be pierced with a fork (begin to check at 10 minutes).  

Take off heat.  Remove bouquet garni and discard.  Stir in horseradish.

Spread potato mixture in buttered dish and sprinkle with cheese.  Bake until top is golden brown and potatoes are tender, 30-40 minutes.  Let stand 15 minutes before serving.