Step 1In a small bowl, add the salt to the water and stir to dissolve. Keep very cold until ready to use.
Step 2To make the dough in a food processor, put the flour in the work bowl. Cut the butter into 1-inch pieces and scatter the pieces over the flour. Pulse briefly until the mixture forms large crumbs and some of the butter is still in pieces the size of peas. Add the water-salt mixture and pulse for several seconds until the dough begins to come together in a ball but is not completely smooth. You should still be able to see some butter chunks.
Step 3To make the dough by hand, put the flour in a mixing bowl. Cut the butter into 1-inch pieces and scatter the pieces over the flour. Using a pastry blender, cut the butter into the flour until the mixture forms large crumbs and some of the butter is still in pieces the size of peas. Drizzle in the water-salt mixture and toss with a fork until the dough begins to resemble a shaggy mass. Gently mix until the dough comes together into a ball but is not completely smooth. You should still be able to see some butter chunks.
Step 4On a lightly floured surface, divide the dough into 2 equal balls and shape each ball into a 1-inch-thick disk. Wrap well in plastic wrap and chill for at least 2 hours or overnight. (Meanwhile, if you haven't already, prepare the lemons.)
Step 5On a lightly floured surface, roll out one disk of dough to one-eighth-inch thick, rolling from the center toward the edge in all directions. Lift and rotate the dough a quarter turn every few strokes to discourage sticking, and work quickly to prevent the dough from becoming warm. Lightly dust the work surface with extra flour as needed to prevent sticking.
Step 6Roll the dough to make a round 1 1/2 inches larger than the pan. Carefully transfer the round to a 9-inch tart pan with a removable bottom (folding it in half or into quarters to simplify the transfer if necessary), easing it into the bottom and sides and then pressing gently into place, leaving a 1-inch overhang. Roll out the remaining disk to make a second round for the top crust. Cover and refrigerate.
Step 1Cut the lemons into paper-thin slices, discarding the thicker stem end and any seeds. Put them in a nonreactive bowl (stainless steel or glass), add the sugar, and toss. Cover and let sit at room temperature for at least 3 hours or overnight. If any seeds are still left, they will usually float to the top, where they are easily fished out. If you are using the more tender-skinned Meyer lemons, you can proceed to the next step without letting them sit, as the skins don't need the sugar to tenderize.
Step 2In a small bowl, whisk the eggs and salt until blended. Add the eggs to the lemon mixture; mix thoroughly. Pour the mixture into the pastry lined tart pan. The mixture will be very liquidy, so distribute the lemon pieces evenly by hand.
Step 3To make the egg wash, in a small bowl, whisk together the egg yolk and cream. Brush the rim of the pastry with the egg wash and lay the second pastry round over the filling. Trim the pastry by pressing down on the edge of the pan and discarding the cut-off scraps.
Step 4Brush the top crust with the egg wash and sprinkle the sanding sugar evenly over the top. Chill for about 30 minutes. Heat the oven to 350 degrees. Cut a few slits in the crust and place the tart on a baking sheet lined with aluminum foil.
Step 5Bake until the pie is golden brown on top and the filling is bubbling (visible through the vents), about 60 minutes.
Step 6Let the pie cool completely to allow the filling to set properly before removing it from the tart pan and slicing. Serve at room temperature or slightly warmed.