Thaw package of puff pastry in the refrigerator (or on kitchen counter for about an hour).
Preheat oven to 400 degrees and line a baking sheet with parchment (or foil).
Cut potatoes into evenly-sized pieces, place in a medium sauce pan and cover with cold water. Bring to boil and cook until fork-tender. Drain water and mash with a hand masher. Add salt and pepper, to taste, along with some butter (if desired). Set aside until ready to use.
In a non-stick skillet over medium heat, add 2 Tb. butter, chopped onion and celery. Cook several minutes until onion becomes translucent, and celery is tender. When done, transfer to the mashed potatoes.
In the same skillet, return heat to medium/high and add the salmon. Season with a little salt and pepper. After about 2 minutes on the first side, then flip, cover the pan and turn off heat (this will allow the fish to continue cooking, while retaining its moisture) and let it sit for 3-5 minutes. (Note: you could also poach the fish in water or broth, or roast it in the oven - whichever you prefer).
Remove cooked fish from the pan and allow it cool slightly before breaking into small pieces/flakes. Add it to the potatoes and stir until everything is combined.
Place puff pastry sheets onto a lightly floured surface. Use a rolling pin, gently roll the pastry to expand it by about 1" all around, then cut each sheet into 4 pieces.
Spoon equal parts of potato mixture onto 4 of the pastry pieces, leaving about 1/2" all around the square.
Dab a bit of water on the edges of the bottom pastry, then place another square on top of the filling. Gently press edges together, then use a fork to crimp the edges.
Place the 4 pouches onto the parchment paper then brush the tops with whatever you have on hand -- a little whole milk, some heavy cream, or 1 egg whisked with a little water.
Place in oven about 30 minutes, until nicely brown. Remove, then allow to cool at least 10 minutes before serving.