1. Dot each fillet with butter. Sprinkle with salt and pepper and fold in half crosswise.
2. Put fillets in a steamer and cook for 5 to 7 minutes. Remove and cool slightly.
3. When fillets are cool enough to handle, wrap each in a spinach leaf. Secure leaf with a toothpick if necessary. Return fillets to the steamer and cook until spinach leaves are wilted but still bright green, about 2 minutes. Serve at once with lemon wedges.