In a small bowl, stir together the paprika, thyme, oregano, salt, and sugar. Pat the fish dry, then sprinkle both sides of it with the spice mixture. The fish should be entirely coated.
In a cast-iron pan or skillet, heat the oil over medium-high heat until it shimmers. Add the butter and wait until it stops foaming. Then add the fillets and let them cook without bothering them, except for flipping halfway through -- after about three minutes. They should take between six and eight minutes to cook in total. Serve as soon as possible.