Cut salmon into four 6 oz portions. Sprinkle each with salt and pepper. Place skin side down on the baking sheet.
Combine panko, cranberries, onion, 2 T. melted butter and lemon peel in a medium bowl and blend well. Season with salt and pepper. Spoon onto salmon, dividing equally. Press to adhere. Drizzle with remaining butter.
Bake until topping is golden and salmon is just cooked, about 20 minutes.