Combine seasoning mix from paprika - onion powder in a small bowl. Generously sprinkle seasoning on both sides of the fish, gently rubbing into the flesh. Let sit for 15 minutes at room temperature.

Heat cast iron pan until very hot over high heat. Add 1 tablespoon oil. Place filet into the pan without overcrowding and cook until charred, about 2 minutes. Flip and cook on the other side until charred and flesh easily flakes. Sprinkle with lemon juice and garnish with cilantro if desired.