In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, cayenne, thyme, oregano, sea salt, and black pepper to create the Cajun rub.
Pat the catfish fillets completely dry with a paper towel and coat both sides generously with the spice mixture, pressing it in to adhere.
Heat 0.5 tbsp of avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Carefully place the catfish in the skillet and sear for 3-4 minutes per side until a dark crust forms and the fish is opaque and flaky.
Remove the fish from the skillet and set aside on a warm plate.
Add the remaining 0.5 tbsp of avocado oil to the same skillet, then add the diced onion, bell pepper, and celery.
Sauté the vegetables for 4-5 minutes until softened, then stir in the cauliflower rice.
Cook the mixture for another 3 minutes until the cauliflower rice is tender and has absorbed the remaining spices from the pan.
Stir in the lemon juice, then divide the cauliflower rice between plates and top with the blackened catfish fillets.