Cut the parchment into two segments — one segment per fish. The size of the parchment depends on the size of your fillets, but you should be fine with a piece of parchment that measures approximately 13" wide.Lay each segment down on a flat surface. Place one fillet in the center of each parchment paper (skin side down if there's skin). Slather 1 TBSP of pesto over each fillet, then top with tomatoes and onions. Drizzle the white wine over the fillets and drop 1/2 tsp of butter onto each fillet.
Once everything is in place, take one of the sides of your parchment and fold it over the fish. After that, grab the other side, and fold it over enclosing the fish. Twist the ends of the parchment one at a time, securing each one by tying butchers twine around it. (See notes for very specific instructions for making parchment packs.)
Place the parchment packs onto a baking sheet and bake for 12-15 minutes, or until the fish is opaque, flaky, and has an internal temperature of 145° F.