1. Preheat your oven to 375°F (190°C).

2. Cook the diced bacon in a large skillet over medium heat until crispy, then set it aside, leaving the grease behind in the skillet.

3. Add sliced onions and minced garlic to the skillet and sauté until the onions become translucent.

4. Next, add sliced mushrooms to the skillet, cooking them until they release their liquid and start browning.

5. In a large bowl, combine the cooked bacon, mushroom mixture, grated cheese, and dried basil.

6. In a separate bowl, whisk together heavy cream, egg yolks, and cold water, then add melted butter, flour, and a pinch of salt, whisking until smooth.

7. Pour the cream mixture over the meat and mushroom mixture, stirring until well combined.

8. Roll out puff pastry on a lightly floured surface to a thickness of about 3 mm (1/8 inch).

9. Transfer the pastry to a 23 cm (9-inch) tart pan with a removable bottom, trim the edges, and press it into the corners of the pan.

10. Fill the pastry-lined tart pan with the meat and mushroom mixture, leaving a 2.5 cm (1-inch) border around the edges.

11. Fold the edges of the pastry up over the filling, pressing gently to seal, then brush the pastry with the beaten egg and cut a few slits in the top to allow steam to escape.

12. Bake the tart for 40-45 minutes, or until the pastry is golden brown and the filling is hot and bubbly.

13. Remove the tart from the oven and allow it to cool for 10 minutes before serving.

14. Serve warm, topped with a dollop of crème fraiche (or sour cream) and a sprinkle of chopped fresh parsley if desired.