Gratin of Apples Onion and Gruyère

Description
I obtained this recipe from an estate sale when I purchased the family collection from the Monsey Estate in Crandall, Texas in 1991.
 * Contributed by Catsrecipes Y-Group

Ingredients

 * ¾ cup flour
 * ½ teaspoon granulated sugar
 * ¼ teaspoon salt
 * 5 tablespoons butter
 * 1 tablespoon cold water
 * 1 egg yolk
 * 2 tablespoons butter
 * 1 medium yellow onion
 * 1 Granny Smith apple cored and thinly sliced
 * 8 slices wine sap apple thinly sliced
 * 4 slices gruyère cheese thinly sliced
 * 2 tablespoons finely chopped fresh chives

Directions

 * 1) In bowl of food processor fitted with chopping blade combine flour, sugar and salt then cut in butter and process mixture until butter is finely distributed.
 * 2) Add water and egg yolk then process until dough forms a ball.
 * 3) Heat oven to 400°F.
 * 4) Divide dough into 4 pieces on lightly floured surface and roll out each piece to a 5-inch round.
 * 5) Fit each round into a 4-inch tart pan.
 * 6) Bake 15 minutes then reduce temperature to 350°F.
 * 7) Melt 1 tablespoon butter in skillet then add onions and sauté 10 minutes covered stirring occasionally then set aside.
 * 8) Melt remaining butter then add all apple slices and sauté until tender and golden.
 * 9) Fill each shell with sautéed onions and top with apple slices arranged in a fan pattern alternating two wine sap slices among the granny smith apples in each tart.
 * 10) Cover apple fans with cheese and bake 5 minutes.
 * 11) Garnish tarts with chopped chives.