Remove chilled dough from fridge and roll into a large rectangle on a lightly floured surface. 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. You can trim any rough edges off, but you do want to be able to cut at least a 16x16" square with straight edges all the way around in the middle of the dough. Work quickly to keep the dough chilled — the thinner it gets, the warmer it will become and you don't want the butter to melt.
Divide the dough in half so that each half is 8x16". Place one sheet of dough on a parchment or silicone mat lined sheet pan in the fridge to chill while you work with the other half.
Cut the remaining sheet of dough in half so that you have two 8x8" squares. On one of the sheets of dough, use a ruler to lightly divide the dough into quarters without cutting through it. So each 8x8" square will have lines dividing it into 4x4" squares. This sheet of dough will be the bottom of your tuna melt hand pies.
Brush the egg wash along the outer edges of the dough and down the center lines. Spoon the chilled tuna filling into the center of each square, maybe 2-3 tablespoon per square.
Then, take the other 8x8" square of dough and gently drape it over the tuna. Start in the center of the dough and use the sides of your hands and flat fingers to press it down around the tuna filling, pushing out any air bubbles and being sure not to squish out any of the tuna. You should have about 1-1½" of flat dough running around each of the tuna pockets.
Use a pizza cutter to divide the dough into quarters, separating the tuna pockets.
Use the tines of a fork to tightly seal the two sheets of pastry together by pressing down all around the edges of the dough right up to where the tuna filling starts. 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 one or 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 salt or everything bagel topping.
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 wrapped tightly in foil or saran wrap in an airtight container in the fridge for up to a week.