Heat the oven to 400 F. Lightly coat a baking dish with cooking spray.
In a skillet, heat the olive oil over medium heat. Add the spinach, garlic and pepper. Saute until the spinach starts to wilt, 2 to 3 minutes.
Place the sole fillets in the prepared baking dish. Place half of the spinach mixture in the middle of each fillet and roll up. Place the rolled fillets seam-side down. Brush with melted butter. Bake until the fish is opaque throughout when tested with the tip of a knife, 8 to 10 minutes. Transfer to individual plates and serve immediately.