Preheat oven to 400 degrees F. Finely chop four green onions. In a small bowl combine chopped green onions, lemongrass, ginger, and oil; set aside. Cut four 15-inch-long pieces of parchment paper. Fold each in half like a book, creasing the center. Open papers and spread an even amount of the lemongrass mixture across the middle of one side of each paper.
Cut the remaining green onions into 2-inch pieces. Top lemongrass mixture evenly with green onions, mushrooms, and peppers. Drizzle lightly with sriracha. Place fish on vegetables; add a squeeze of lime juice. Season with salt. Top with cilantro.
Fold parchment over food. Seal packets: Starting at the bottom folded edge, crease the corner up at an angle. Working in 2-inch sections, continue creasing open edges, overlapping each fold. Make sure folds are creased tightly so they don't open. Finish by tucking the last flap underneath packet (packet will be a half-moon shape). Transfer filled packets to a baking sheet. Bake 15 to 20 minutes.* Make a narrow slit and check if fish flakes easily when tested with a fork. (Bake 1 to 2 minutes more, if needed.) Open packets carefully to serve. Makes 4 servings. *Tip: If fish fillets are 3/4 to 1 inch thick, increase baking time to 20 to 25 minutes.