Get started by preheating your oven to a comfortable 400 degrees F (204 degrees C). While it's warming up, gently pat your flounder fillets completely dry with paper towels. Arrange them in a single layer on a rimmed baking sheet or in a baking dish, making sure they don't overlap.
In a small bowl, whisk together the melted butter, olive oil, and lemon juice until well combined. Use a brush to lightly coat both sides of each flounder fillet with this delicious mixture.
In another small bowl, mix the smoked paprika, garlic powder, sea salt, and black pepper. Sprinkle this seasoning blend generously over both sides of the prepared fish.
Pop the seasoned flounder into the preheated oven and bake for about 12 to 15 minutes. The fish is perfectly cooked when it easily flakes apart with a fork, or when its internal temperature reaches a tender 140 degrees F (60 degrees C). Keep an eye on it around the 10-minute mark, as cooking times can vary based on fillet thickness.