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