In a small bowl, stir together garlic powder, salt, paprika, red pepper, and thyme. Lightly brush fish with 2 tablespoons oil. Sprinkle both sides of fish with garlic powder mixture.
In a large cast-iron skillet, heat remaining 2 tablespoons oil over medium-high heat. Add fish; cook until fish can be easily turned with a spatula, about 3 minutes. Turn fish; cook until fish is opaque and flakes easily, about 2 minutes more.
Spread Spicy-Sweet Tartar Sauce over cut sides of bread. Top with fish, lettuce, and tomato. Serve with potato chips.
Recipe by Southern Cast Iron at https://southerncastiron.com/blackened-catfish-po-boys/