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, dried thyme, dried oregano, cayenne pepper, sea salt, and black pepper.
Generously coat both sides of the catfish fillets with the spice rub, pressing it into the flesh with your fingers.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Add the catfish fillets to the hot skillet and cook for 3 to 4 minutes per side until the exterior is blackened and the fish is opaque and flaky.
Remove the fish from the skillet and set aside on a plate; in the same skillet, add the sliced bell peppers and red onion.
Sauté the vegetables for 4 to 5 minutes until they are tender and slightly charred, then stir in the cooked quinoa to warm it through.
Serve the blackened catfish over the bed of sautéed vegetables and quinoa, garnished with fresh parsley and a squeeze of lemon.