Preheat oven to 400° F. Place a piece of parchment paper on a 13-by-12-inch baking sheet. Put the potatoes on the sheet and season with 1 teaspoon of the salt and ½ teaspoon of the pepper. Distribute the butter over the potatoes. Roast in top of oven for 45 to 50 minutes or until done. Meanwhile, lay out 4 sheets of parchment (at least 13 by 13 inches) and place 1 fillet on each. Top with the asparagus, scallions, and tarragon. Drizzle with the oil and season with the remaining salt and pepper. Top each fillet with a lemon half, after first squeezing on the juice. Pull the sides of the paper over each fillet, folding several times to seal. Fold and twist the ends to form rectangular parcels. Place them on a sheet pan. Bake on lower rack of over for 20 to 25 minutes while the potatoes finish cooking. Place a salmon parcel on each plate and cut open. Serve with the potatoes.