- 1 1/2 cups unbleached all purpose flour
- 1/2 cup (1 stick) chilled unsalted butter, cut into pieces
- 1/4 cup (1/2 stick) unsalted butter
- 1 3/4 pounds onions, thinly sliced
- 1 cup grated Blarney cheese, Fontina cheese or Swiss cheese (about 4 ounces)
Blend flour and salt in processor. Add butter and blend in using on/off turns until mixture resembles coarse meal. Mix sour cream and egg yolk together. Add to flour mixture and blend in using on/off turns just until dough begins to form clumps. Gather dough into ball; flatten into disk and refrigerate 30 minutes.
Preheat oven to 425°F. Roll out dough on lightly floured surface to 13-inch round. Transfer dough to 11-inch-diameter tart pan with removable bottom. Trip edges. Line crust with foil. Fill with dried beans or pie weights. Bake 15 minutes. Remove foil and beans. Continue to bake until pale golden, piercing crust in pan bottom with fork if dough bubbles, about 15 minutes. Transfer to rack and cool.
Melt butter in heavy large skillet over medium-low heat. Add onions and sauté until very soft and golden, stirring often, about 45 minutes. Season to taste with salt and pepper; cool. DO AHEAD Crust and onions can be prepared 1 day ahead. Cover crust and let stand at room temperature. Cover onions and refrigerate.
Preheat oven to 325°F. Whisk half and half, yolks and egg in medium bowl. Season to taste with nutmeg, salt and pepper. Spread onions in crust. Sprinkle with cheese. Ladle half and half mixture over. Bake tart until filling puffs and is golden, about 50 minutes. Cool slightly. Serve warm or at room temperature.