Carrot juice is packed with beta-carotene, iron, and potassium and makes a nice addition to sauces and dressings.
Preheat oven to 375 degrees. Coat a rimmed baking sheet with nonstick cooking spray. Season snapper fillets with salt and pepper; place flat side down on sheet. Bake until opaque, 12 to 15 minutes.
Meanwhile, in a small skillet, bring carrot juice to a boil. Reduce heat; simmer until reduced by half, 5 to 8 minutes. Remove from heat; let cool slightly.
Season with salt and a pinch of cayenne pepper. Add lemon juice and butter, cut into small pieces. Stir with a wooden spoon until butter is incorporated. Sprinkle fish with scallion; serve with sauce.