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