Stir the flours, sugar and salt together in a large bowl. Rub in the butter to create flat flakes, keeping it nice and cold, until the flakes are about the size of your pinky nail.
Add the water all at once, tossing it in at first and folding it into the dry ingredients until it just forms a shaggy, loose dough.
Sprinkle a touch of flour on your work surface and tap the dough into a rectangle about 10” by 6”. Fold in in thirds, repeating if the dough is cold enough. Wrap and chill for at least one hour.
Filling
Stir the lemon and salt into the crème fraiche and set aside.
Heat a large skillet and add in the butter, allowing it to melt and just begin to sizzle. Place the cut leeks open-face down into the butter, followed by the shallots. Cook until just getting some color. Flip the leeks over and cook to brown the other side. Turn the heat off and squeeze the lemon into the pan. Salt to taste.
Assembly
Lightly flour your work surface. Roll out your pie dough into a large circle and trim to 14” around. Place this dough on a parchment-paper lined cookie sheet.
Sprinkle half the Gruyere over the bottom of the dough. Spread the crème fraiche over the cheese and top with cooked leeks and shallots, in any pattern of your choosing leaving 2” all around the perimeter of the circle without any filling.
Fold the 2” overhang over the fruit with 5 to 6 folds around the circle. Lightly brush the outside folds with milk and, if desired, sprinkle cheese over the milk. Place in the oven and turn it down immediately to 375F. Bake for about 25 minutes, or until you reach a lovely golden color throughout.