Cut the salmon portions in half, season with salt, pepper, and fresh dill, squeeze lemon juice over them, and marinate for about 15 minutes.

Melt butter in a large skillet over medium heat, add shallots and leeks, and cook until tender (about 5-8 minutes). Add white wine, cook for an additional 2 minutes, then stir in Dijon mustard, ⅓ cup mascarpone, and season with salt and pepper. Set aside.

Unfold puff pastry onto a baking sheet lined with parchment paper and preheat oven to 400°F (200°C).

Spread the remaining mascarpone in a strip down the center of the puff pastry, leaving margins. Season with salt, pepper, and sprinkle fresh dill. Layer the marinated salmon, adding more salt, pink peppercorn, fresh dill, and lemon zest, then top with the leek mixture.

Cut the puff pastry sides into parallel diagonal strips, about 0.6-inch wide. Fold the top and bottom over the filling, then alternate folding the strips over to cover the filling.

Whisk the egg with 1 Tablespoon water and a pinch of salt. Brush the puff pastry with the egg wash and sprinkle with sesame seeds.

Transfer the filled pastry onto the preheated baking sheet and bake for about 25 minutes, or until golden brown. Let cool for about 10 minutes before slicing and serving.