In a small bowl, whisk together the Greek yogurt, lemon juice, minced garlic, and Dijon mustard to create the aioli, then set aside in the refrigerator.
In a shallow dish, combine the smoked paprika, garlic powder, onion powder, dried oregano, dried thyme, cayenne pepper, sea salt, and black pepper.
Pat the catfish fillets completely dry with paper towels and dredge both sides thoroughly in the spice mixture, pressing gently to adhere.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it is shimmering and very hot.
Carefully place the catfish in the skillet and sear for 3 to 4 minutes per side until the spice crust is dark and the fish flakes easily with a fork.
While the fish is searing, steam the green beans in a small amount of water for 4 to 5 minutes until they are vibrant green and crisp-tender.
Plate the blackened catfish immediately, topped with a generous dollop of the lemon aioli and served with the steamed green beans on the side.