Thaw the pastry overnight in the refrigerator, or according to package directions. (Take your cream cheese out of the fridge too, you'll need it soft.) Keep the pastry chilled until you're ready to roll it out. Line two baking sheets with parchment and set aside.
In a large bowl, stir together the cream cheese, sour cream and mayonnaise, garlic, parmesan, gruyere, mozzarella, hot sauce, Worcestershire sauce, salt and pepper until fully combined.
Squeeze out any excess liquid from the artichokes and spinach and stir them in, along with the shredded chicken. (The filling can be made a day or two ahead.)
In a small bowl, beat the egg with 2 teaspoons of water and set aside.
Place a 10" X 15" puff pastry sheet on a floured work surface and lightly roll it out to about 12” x 17”. Trim the edges if you want. Cut the long side into thirds and the short side in half to make six equal rectangles.
Mound about 3/4 cup (200g) of the mixture (if making 8 pies; use about a cup if making 6 pies) in the center of half the pastry sheets, spreading it out evenly but leaving about a 1-inch clean border along all four sides. Brush borders of pastry with egg wash. Place the top rectangles over the filling, lining up the edges as best you can. Press edges with a fork to seal them shut. Repeat the last two steps with the rest of the pastry sheets to make all your pies.
Carefully move the pies to the baking sheets and refrigerate for at least 20 minutes and up to a day (cover in plastic wrap if chilling overnight).
Heat oven to 400ºF and set a rack in the middle (if both sheets don’t fit on one rack, use upper and lower racks, and swap trays halfway through baking). Lightly brush tops of the pies with egg wash, and sprinkle with sesame seeds or poppy seeds, if desired. Use a sharp knife to cut 3 small slits in the top of each pie to let the steam escape.
Bake until the pastry is puffed, golden brown and cooked through, 30 to 35 minutes. Let cool for 10 minutes on baking sheet. Serve warm or at room temperature.