In a small bowl, combine the smoked paprika, garlic powder, onion powder, dried thyme, dried oregano, cayenne pepper, sea salt, and black pepper to create the blackening spice rub.
Pat the catfish fillets completely dry with paper towels to ensure a proper sear, then coat both sides of the fillets generously with the spice mixture.
Heat the avocado oil in a large cast-iron skillet or heavy-bottomed pan over medium-high heat until the oil is shimmering and just starting to smoke.
Carefully place the catfish fillets in the hot skillet and cook for 3 to 4 minutes per side, or until the spices have formed a dark crust and the fish flakes easily with a fork.
Remove the fish from the skillet and set aside on a plate to rest.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 4 to 5 minutes until they are tender-crisp and have picked up the residual spices from the pan.
Serve the blackened catfish over the sautéed vegetables and finish with a fresh squeeze of lemon juice.