In a small bowl, whisk together the Greek yogurt, mayonnaise, lemon juice, lemon zest, and minced garlic to create the aioli, then set aside in the refrigerator.
In a separate small bowl, combine the smoked paprika, garlic powder, onion powder, dried thyme, dried oregano, cayenne pepper, sea salt, and black pepper to create the blackening seasoning.
Pat the catfish fillets completely dry with a paper towel and coat both sides generously with the spice blend, pressing it gently into the flesh.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it is shimmering and very hot.
Carefully place the catfish fillets in the skillet and sear for 3 to 4 minutes per side until a dark crust forms 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 tender-crisp and bright green.
Plate the blackened catfish immediately with a dollop of the chilled lemon aioli and serve alongside the steamed green beans.