Thaw the puff pastry by placing one sheet in the refrigerator for about 4 hours or overnight.

Prepare the Boursin cheese by softening it at room temperature for 30 minutes or microwaving for 10-15 seconds.

Wash and slice tomatoes into ¼-inch thick rounds and sprinkle with kosher salt, allowing them to sit for 20 minutes.

Roll out the thawed puff pastry to ⅛ inch thick and place on a parchment-lined baking sheet, scoring a ¾-inch border around the edges.

Spread the softened Boursin cheese in the center of the pastry within the scored border, add chopped basil, and layer the tomato slices.

Chill the assembled tart in the freezer for 20 minutes while preheating the oven to 400°F (200°C).

Drizzle olive oil over the tomatoes and season with black pepper.

Mix egg yolk with water and a pinch of salt for the egg wash, applying it to the pastry border.

Bake the tart for 10 minutes, then add remaining Boursin cheese and continue baking for 20-25 minutes until golden brown.

Allow to cool for a few minutes before slicing and serve warm or at room temperature garnished with fresh basil.