In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, dried thyme, dried oregano, cayenne pepper, sea salt, and black pepper.
Pat the catfish fillets dry with a paper towel and press the spice rub firmly onto both sides of the fish to create a thick, even coating.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until the oil is shimmering and very hot.
Carefully place the catfish in the skillet and sear for 3 to 4 minutes per side until a dark, crispy crust forms and the fish flakes easily with a fork.
Remove the fish from the skillet and set aside; in the same pan, add the sliced bell peppers and onions, sautéing for 5 minutes until tender and slightly charred.
Serve the blackened catfish over the cooked quinoa, top with the sautéed vegetables, and finish with a bright squeeze of fresh lemon juice.