- With the rack in the lowest position, preheat the oven to 400°F (200°C). Line a baking sheet with a silicone mat or parchment paper.
- On a lightly floured surface, roll out the puff pastry into a 7 x 14-inch (18 x 35 cm) rectangle. Trim the edges so that they are straight. Using the tip of a knife, score a smaller rectangle into the dough 1/2 inch (1 cm) inside the edges. Place on the prepared sheet and refrigerate for 15 minutes.
- Meanwhile, in a bowl, combine the grated Parmesan with the cream. Let rest for 10 minutes.
- Spread the Parmesan mixture onto the inner rectangle of the dough. Top with the endive, shallot and prosciutto.
- Bake for 20 minutes or until the pastry is golden brown.
- Meanwhile, in a bowl, toss all the ingredients together. Season with salt. Garnish the tart with the watercress mixture, then cut into pieces and serve as an appetizer.