Potato and Root vegetable Gratin

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Serves: 4
Cal: 768
By: Fernando Divina, Tendrils Restaurant, Cave B Inn at SageCliffe and The Center for American Food and Wine at SageCliffe, Quincy, Washington

Preheat the oven to 425°F.

Cream 1/4 cup butter with ground mustard seed and grated mustard root. Coat bottom and sides of a shallow 2-quart ceramic or glass casserole dish with the mustard flavored butter.
Mix potatoes and celery root in a bowl and place mixture in baking dish. Level evenly over bottom and tamp down slightly.
Beat egg yolks and whole eggs with a fork to blend thoroughly. Add heavy cream, salt, and pepper to eggs and mix well. Pour egg mixture over potato mixture. Dot top of the casserole with the remaining 2 tablespoons of butter. Bake in the middle of a 425° F oven for approximately 30 minutes or until golden brown.
Turn oven down to 350° F, remove casserole and loosely cover with foil. Return the casserole to the oven and bake another 12 to 20 minutes or until vegetables are tender when pierced with the tip of a knife and knife comes away clean, indicating custard is set. Allow finished casserole to stand for 10 to 15 minutes before serving for flavors to ripen and gratin to firm, making serving easier.

1/4 cup plus 2 tablespoons unsalted butter at room temperature
1/2 teaspoon dry mustard seed powder
4 ounces mustard root or horseradish root, peeled and finely grated (about 3-4 tablespoons grated)
2 medium Washington Russet potatoes, peeled and thinly sliced
1/2 pound Washington Provento or Yellow Finn potatoes with skin, washed and thinly sliced
1/2 pound Washington Chieftain or Red LaSoda waxy potatoes, washed and thinly sliced
1/2 pound celery root, peeled and thinly sliced*
2 egg yolks
2 whole eggs
2 cups heavy cream
1-1/2 teaspoons coarse salt
1/4 teaspoon freshly ground pepper
*Sunchokes and 2 tablespoons minced celery may be substituted.