Preheat oven to 400F. Make an egg wash by beating egg with 1 Tbsp. water in a small bowl. This will help make your pastry brown and shiny.

Lightly flour work surface and Rolling Pin. If using a 14-oz puff pastry package, roll the sheet to a 11x15-inch rectangle. If using a 17-oz puff pastry package, line up the two longer sides and overlap by 1 inch to form a 11x15-inch rectangle. Brush egg wash in between to form a seam. Use a Rolling Pin to seal the seam. Trim sides of pastry if necessary to create a straight-sided 11x15-inch rectangle.

Transfer pastry to a Baking Slab. Fold sides of pastry towards center to form an even border on all four sides. Lightly score the border using a Paring Knife to make small shallow diagonal cuts. Prick the surface of the pastry inside the border with a fork.

Use a pastry brush to brush the border and docked pastry with egg wash. Bake until puffed and very lightly browned, about 10 minutes. Let cool slightly.

Brush base of pastry (but not sides) with mustard. Sprinkle with half the cheese. Layer the slices of mortadella or prosciutto on top (if using), and top with the rest of the cheese. Trim asparagus to fit inside pastry borders and toss with a little olive oil, salt, and pepper. Arrange spears on top of tart. Add garnishes of choice, drizzle with olive oil, and season with salt and pepper.

Bake tart until golden brown and asparagus is cooked through, 25–30 minutes. Let cool slightly, then top with lemon zest and herbs, if using. Cut into squares and serve.