Line a baking sheet with nonstick aluminum foil and place it in the oven. Preheat the oven to 400 degrees F.
For the butter: Meanwhile, whisk together the butter, lemon zest and juice, capers, garlic and salt in a medium bowl.
For the sheet: Slice the spring onions in half lengthwise. If using scallions, they can be left whole.
Make sure you have all of your ingredients ready to go so you can assemble the sheet and return it to the oven quickly. Remove the sheet from the oven once the oven is preheated. Quickly add the fish in a row, leaving about an inch between each piece. Drizzle the butter mixture over the top, add the lemon slices over the fish, then the spring onions. Sprinkle with paprika. Return the sheet to the oven and bake until cooked through, 12 to 18 minutes. Serve warm, preferably with garlicky mashed potatoes!