In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, dried oregano, dried thyme, cayenne pepper, sea salt, and black pepper to create the Cajun seasoning.
Pat the catfish fillet completely dry with a paper towel on both sides to ensure a crispy crust.
Gently press the Cajun seasoning onto both sides of the catfish fillet until it is evenly and thoroughly coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until the oil is shimmering and hot.
Add the sliced red bell pepper, green bell pepper, and red onion to the skillet and sauté for 4 minutes until they are tender-crisp and slightly charred.
Push the vegetables to the edges of the pan and place the seasoned catfish fillet in the center.
Sear the catfish for 3 to 4 minutes per side without moving it, until a dark, crispy crust forms and the fish is opaque and flaky.
Place the warm cooked quinoa in a bowl, top with the blackened catfish, and garnish with the sautéed peppers and onions.