01 - Preheat the oven to 400°F. Roll out the puff pastry and line a 9-inch tart pan, trimming excess. Prick the base with a fork.
02 - Spread Dijon mustard evenly over the base of the pastry.
03 - In a bowl, whisk together the egg, crème fraîche (or heavy cream), half of the Parmesan, chopped basil, salt, and pepper. Pour this mixture over the pastry base.
04 - Arrange mozzarella slices evenly over the filling, then layer tomato slices attractively on top. Drizzle with olive oil.
05 - Sprinkle the remaining Parmesan over the tomatoes.
06 - Bake for 30–35 minutes, or until the pastry is golden and the filling is set.
07 - Let cool slightly before garnishing with additional fresh basil. Serve warm or at room temperature.