In a small bowl, whisk together the Greek yogurt, lemon juice, minced garlic, and Dijon mustard until smooth to create the aioli, then set aside.
Pat the catfish fillets completely dry with paper towels to ensure a proper sear.
In a shallow dish, combine the smoked paprika, garlic powder, onion powder, cayenne, thyme, oregano, sea salt, and black pepper.
Firmly press both sides of each catfish fillet into the spice mixture until they are thoroughly and evenly coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Carefully place the catfish fillets in the skillet and sear for 3-4 minutes per side until the spice crust is dark and the fish flakes easily with a fork.
While the fish cooks, steam the green beans in a small amount of water for 4-5 minutes until they are bright green and crisp-tender.
Serve the blackened catfish immediately alongside the steamed green beans and a generous dollop of the lemon aioli.