Heat a black iron skillet over high heat until it smokes all by itself. Turn the exhaust fan on.
While the skillet is heating, pass the fillets through the melted butter, and shake off the excess. Then generously sprinkle the blackened seasoning on both sides of each fillet (you can even dredge it through the seasoning if you like).
Place the fish in the skillet and sear it for a minute or two per side, depending on thickness. (Check to make sure the fish is done by piercing the thickest part with a kitchen fork. Hold it in the center for a few seconds, then carefully touch the fork to your lips. If the fork seems even a little warm, the fish is done.)
Let the fish cool for a few minutes, then cut into strips or large chunks.
Mix the feta cheese into the dressing. Toss the greens with the dressing, then place on a salad plate. Top with the asparagus, tomatoes, and catfish. Drizzle a little more dressing and lemon juice over all.