Puff crust dough: grate the frozen butter onto parchment paper, and once it's fully grated, move it back into the freezer to be used later. Keeping the butter very cold is very important in this recipe. 

Add the rest of the dough ingredients to a large bowl and stir to fully combine. Once there are no dry spots, using your hands, knead the dough for 2 to 3 minutes. Shape the dough into a rough ball, cover the bowl with a lid or plastic wrap, and allow the dough to rest and rise for 30 minutes. 

Remove the dough from the bowl and move it to a lightly floured countertop or surface. Roll the dough into a rough 5 x 7-inch rectangle. 

Sprinkle on 1/3 of the frozen, grated butter. Lightly press the butter with your fingertips so that it's stuck to the dough. Now you're going to perform a letter fold. Imagine the dough rectangle split into thirds (the long way). Fold the left third over the middle third, and then fold the right third over on top of the middle. Check out the blog post that details the lamination process with photos if this isn't immediately clear. 

Now you should have a long, skinny rectangle that's around 1.5 to 2 inches wide. Using your hands or a rolling pin, flatten that rectangle of dough until it's about 4 inches wide. Evenly sprinkle on the rest of the butter. 

Again, imagine that the dough is split into thirds, this time the short way. Fold the bottom third up over the middle third, and then fold the top third to cover the rest of the folded dough. Again, check that blog post linked above if you can't fully imagine the scenario. 

Roll the folded dough into a thin rectangle that's as close to 9 x 13 inches as you can get it. Cover the dough and let it rest and rise for 30 to 45 minutes. 

After 30 to 45 minutes, the dough should be a little bit puffy, and it should be ready to bake. If you do not want to bake at this time, you can store the dough on a sheet pan in the fridge, covered, for 1 to 2 days. You will need to remove it from the fridge at least 30 minutes before you want to bake it so the dough can warm up a bit. 

If you're ready to bake, cut the dough rectangle in half and preheat your oven to 475°F (250 C). This dough should make 2 sandwiches, and that's why you cut it in half, but if you want smaller sandwiches, you could cut it in thirds. Place each dough rectangle that you plan to bake on a sheet pan. 

Parmesan and panko topping: while the oven is preheating, add all of the Parmesan and panko topping ingredients to a bowl and stir to combine. 

Sandwich assembly and bake: spread a tablespoon or two of Dijon mustard on the bottom half of each piece of dough that you are cooking. Top the mustard with an even layer of shredded Swiss cheese. Fold the dough over on top of the cheese to form a pocket. Do not pinch any of the seams closed; we will be opening them again to add the rest of the ingredients later. 

Spread a teaspoon of olive oil on top of the dough pocket and then sprinkle the oil with a liberal amount of the Parmesan and panko mix. The oil will help the dough brown, and it also helps the seasonings to adhere to the top. 

Bake for 10 minutes and then remove the sheet pan from the oven. Carefully fold open each dough pocket. 

Layer on ham, topped with sliced and cooked chicken pieces, and then top with more shredded Swiss cheese. 

Close the pocket and add the sheet pan back into the oven for an additional 5 to 7 minutes to finish baking and warm everything up. 

Remove from the oven, allow to cool for 4 or 5 minutes on a cooling rack, and serve. The cheese will be very hot.