Tart Garnish: Trim and clean the leeks. Cut in ½ lengthwise and slice thinly. Wash apple, then peel and core it. Slice apple thinly, turn and slice into small batons. Heat butter and oil in saute pan. Add leeks and sweat. Add apple pieces and salt and nutmeg to taste. Cook to dry. Cool and refrigerate.
Short Crust: Cut butter into small dice. Sift flour onto block. Cut butter into flour until it is sandy-textured. Make a well in the center of the flour. Beat together the egg, water and salt and pour into the well. Work the liquids into the solids. Form a ball of dough and refrigerate at least half an hour.
Custard: Whisk the eggs and cream together. Season with salt, pepper, nutmeg and cayenne pepper to taste. Refrigerate.
Assembly :Roll the dough and fit into eight small (9 cm) tart rings or one large (26 cm) tart ring. Line with parchment paper, fill with dried beans and blind bake at 350° F for 20 minutes. Remove beans and cool shell to room temperature.
Raise oven temperature to 400° F.
Place a thin layer of grated Gruyere cheese at the bottom of the tart shell(s.) Top with a layer of leek and apple confit. Pour in custard to cover cheese and vegetables. Bake at 400° F for 25-30 minutes for a large tart or 15-20 minutes for small tarts. Cook until crust is golden and custard is set. If necessary, lightly cover the tart with foil after 15 minutes to avoid scorching the exposed vegetables.