Remove chilled puff pastry dough from fridge. Divide in half. Wrap and refrigerate the dough you aren't using.Roll the puff pastry sheet to about ⅛" thickness. Use a bench scraper to square off the edges of the dough as you go, but it's ok if the edges are a little rough or misshapen. Trim the sheet to an 8x16" rectangle. Then divide the sheet in half so you have two 8x8" squares of puff pastry.
Place one sheet on a lined sheet pan in the fridge to chill while you work with the other.
Brush the egg wash along the outer edges of one 8x8" square of dough and through the center in both directions, creating a four section grid on the pastry. Spoon the chilled tuna filling into the center of each square, about 2 tablespoons each. Work quickly so the dough doesn't get too warm or sticky!
Take the other 8x8" square of dough from the fridge and roll it slightly bigger in each direction. Gently drape it over the tuna melt filling. Start in the center of the dough and work outwards, pressing it down around the tuna filling, pushing out any air bubbles without squishing out any of the tuna. You should have about 1" of flat dough running around each of the tuna pockets.
Use a pizza cutter to divide the square into quarters, separating the tuna hand pies. Crimp the edges of each tuna hand pie with a fork to seal them together. Trim away any excess dough around the edges.
Place the four tuna hand pies on a silicone or parchment lined sheet pan and chill in the freezer for 30-40 minutes before baking. Repeat with the remaining dough and tuna filling.
Preheat oven to 375°F while the hand pies are in the freezer. Right before baking, cut two slits in the surface of the pastry (be careful not to cut into the bottom sheet of dough!), brush the hand pies with an egg wash and top with flaky salt.
Bake for 25-35 minutes until golden brown and flaky. Remove from the oven and let cool on a wire rack. Serve warm, room temp, or right out of the fridge. Store extras in an airtight container in the fridge for up to a week.