In a large mixing bowl, combine the flour, sugar, and salt. Using a pastry cutter or your fingertips, cut in the butter until the bits of butter are about the size of a pea. Stir in the egg yolk.
Working with only a teaspoon or two at a time, gradually add just enough water so that when you press the dough together it stay intact. The dough should still be shaggy and crumbly when done. Do not at too much water.
Press the dough into a 9-inch tart pan. Start by pressing the dough up the sides as evenly as possible, then fill in the bottom. Reserve a bit of the dough to repair any cracks that may occur during baking. Chill in the refrigerator for 30 minutes as you pre-heat your oven to 375 degrees.
Once chilled, line the tart pan with foil, shinny-side down. Fill with pie weights or dry beans and bake for 20 minutes. Remove the tart pan from the oven and very carefully take out the weights and foil. At this point, patch up any minor craks with the reserved dough.
Return the tart back to the oven for an additional 5 to 10 minutes, or until the crust is light golden brown. If the center puffs up during baking, gently press is back down with a piece of foil or parchment paper.
Resume recipe for the Grapefruit Lemon Tart, or cool completely before wrapping in plastic. The tart shell may be stored wrapped well in the refrigerator for a couple of days or frozen in the freezer for a few months.