In a mixing bowl, whisk together all-purpose flour and salt. Cut in cold unsalted butter with a pastry cutter until the mixture resembles pea-sized crumbs. Add creme fraiche and gradually incorporate ice-cold water until a thick dough forms. Shape the dough into a disc, wrap it in plastic, and refrigerate for at least 1 hour to chill.

Remove the chilled dough and let it sit at room temperature for about 5-10 minutes. On a lightly floured surface, roll out the dough to about 1/8 inch thick, ensuring it's large enough to fit a 9-inch tart pan. Carefully transfer the dough into the pan, trim the edges, and prick the bottom with a fork to prevent bubbling. Freeze for 1 hour for best results.

Preheat your oven to 400°F (200°C). Line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake for 20-25 minutes until the edges begin to set. Remove the weights and parchment, and bake for an additional 10 minutes, or until the crust turns a light golden brown.

In a skillet, cook thick-cut bacon over medium heat until crispy, then set aside to cool before chopping. In the remaining bacon fat, sauté sliced white onions until they become translucent, approximately 5 minutes. Allow both the bacon and onions to cool completely.

In a medium bowl, whisk together the eggs, remaining creme fraiche, and freshly cracked black pepper. Gently fold in the cooled onions and crispy bacon, ensuring an even distribution.

Pour the filling mixture into the pre-baked tart shell, spreading it evenly. Bake in the preheated oven for 40-45 minutes, or until the filling is bubbling and the crust is a deep golden brown.

Once baked, remove the tart from the oven and allow it to cool for about 30 minutes on a wire rack. Serve warm or at room temperature.