In a small bowl, whisk together the Greek yogurt, minced dill pickles, Dijon mustard, and lemon juice to create the tartar sauce, then set aside in the refrigerator.
In a shallow dish, combine the smoked paprika, garlic powder, onion powder, cayenne pepper, dried thyme, dried oregano, sea salt, and black pepper.
Pat the catfish fillets completely dry with paper towels and coat both sides generously with the prepared Cajun spice rub.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it is shimmering and very hot.
Place the catfish in the skillet and sear for 3 to 4 minutes per side until the crust is dark and charred 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.
Serve the blackened catfish immediately with a dollop of the chilled tartar sauce, the steamed green beans, and a fresh lemon wedge.