Cut two large pieces of parchment paper from the roll or use foil. The pieces should be at least 16-18’ long.
Fold each piece in half lengthwise to mark the center. Unfold and lay flat on a clean workspace. Have ready a rimmed cookie sheet.
Place vegetables (baby spinach, bell peppers, onion rings, and mushrooms) on the bottom, covering one half of the parchment with a 1” border. Drizzle lightly with oliveoil and sprinkle with kosher salt and black pepper.
Lay a filet of fish on top of the vegetables. Drizzle with oliveoil and season with koshersalt,blackpepper, and crushed red pepper flakes (if using).
Begin sealing the parchment by starting in one corner and folding a sealed edge all along the parchment until you’re left with a small hole. Blow gently in to inflate the package a little, then finish twisting the corner to seal the packet. Repeat with remaining parchment, fish, and vegetables.
Bake the packets for 15-20 minutes, or just until vegetables are done and the fish is opaque.
Carefully open the packets, being cautious of rising steam. Gently slide the contents off of the parchment onto a shallow bowl, including all juices. Sprinkle with chopped fresh basil or Italian parsley before serving.