01 - Preheat the oven to 400°F. Roll out the puff pastry and line a 9-inch tart pan, trimming any excess pastry from the edges. Prick the base thoroughly with a fork to prevent air bubbles during baking.
02 - Spread the Dijon mustard evenly over the entire surface of the pastry base using the back of a spoon or small spatula.
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 and smooth.
04 - Pour the cream mixture evenly over the prepared pastry base. Arrange the mozzarella slices in a single layer over the filling, then layer the tomato slices on top in an attractive pattern. Drizzle with olive oil.
05 - Sprinkle the remaining Parmesan cheese generously over the top of the tomato layer.
06 - Bake for 30–35 minutes, or until the pastry is golden brown and crisp, and the filling is set and bubbling around the edges.
07 - Allow the tart to cool slightly for about 5–10 minutes before garnishing with additional fresh basil leaves. Serve warm or at room temperature.