Preheat the oven to 375 degrees. Lightly grease a baking dish with butter, olive oil, or nonstick spray.
Pat the flounder fillets dry with paper towels and lay them flat in the baking dish in a single layer.
In a small bowl, whisk together olive oil (or butter), lemon juice, garlic powder, smoked paprika, salt, and pepper.
Pour the mixture evenly over the fillets, using a spoon or brush to coat the tops.
Bake uncovered for 6 minutes. Sprinkle the grated parmesan on top of the fish.
Bake for an additional 6-8 minutes, or until the fish is opaque and flakes easily with a fork.
Serve the fish drizzled in butter sauce.
