Pat the catfish fillets completely dry with paper towels to ensure the spices adhere and the fish sears properly.
In a small bowl, combine the smoked paprika, garlic powder, onion powder, thyme, oregano, cayenne, sea salt, and black pepper.
Generously coat both sides of the catfish fillets with the spice mixture, pressing it firmly into the flesh.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Place the fillets in the hot skillet and cook for 3 to 4 minutes per side until the crust is dark and the fish flakes easily with a fork.
Remove the catfish from the skillet and set aside on a warm plate.
In the same skillet, add the sliced bell peppers and red onion, sautéing for 5 minutes until they are tender-crisp and slightly charred.
Serve the blackened catfish fillets immediately over the bed of sautéed vegetables.