Combine the flour and salt in a large bowl. Add the cubed butter and, using a pastry cutter (or the tip of your fingers), cut the butter into the flour until the mixture resembles coarse sand. Add the water, one tablespoon at a time, and mix - adding more water as needed - until the dough comes together. Do not overmix!

Turn the dough out onto a lightly floured surface and knead just until it forms a ball. Transfer the dough onto a piece of parchment paper, flatten into a disk, cover with a second piece of parchment paper, and roll into a 10-inch circle.

Carefully transfer the dough to a 10-inch tart pan and chill for 30 minutes, or until firm.

Preheat oven to 425 degrees F.

Using a fork, pierce the bottom of the crust. Cut a square of parchment paper about 4 inches larger than the pie plate. Line the crust with the paper and fill with ceramic pie weights (or dried beans).

Bake in the preheated oven for 10 minutes or until set. Remove from oven and gently remove the paper and weights. Reserve the crust.

In a large skillet, over medium heat, add the diced bacon and cook until crisp and golden brown, about 5 minutes. Using a slotted spoon, remove the bacon to a plate. Reserve.

Discard all but 1 teaspoon of the bacon fat. Add the butter and, once melted, add the chopped shallot. Cook until translucent, about 2 minutes. Add the sliced leeks and stir to coat them with the fat. Add the white wine, partially cover, and cook, stirring occasionally, until the leeks are soft, about 8 to 10 minutes. Season with salt, pepper and nutmeg.

Add back the bacon. Stir in the flour and cook, stirring constantly, for a couple of minutes. Then, add the milk and stir until thickened. Add the creme fraiche and stir until incorporated.

Remove the pan from the heat and let it cool slightly. Then, add the eggs and 1 cup of the Gruyere cheese, stirring until combined.

To assemble the tart, pour the filling mixture into the reserved crust. Scatter the remaining cheese evenly over the top.

Bake until set and nicely browned, about 35 to 40 minutes. Let it cool for at least 30 minutes before serving.

Serve warm or at room temperature.