In a bowl or mason jar, combine the salt, garlic powder, sweet paprika, smoked paprika, thyme, cayenne pepper and black pepper. Stir or shake to incorporate and set aside. Any leftover spice mix can be stored in the sealed jar for up to one month.
Rinse and pat fish very dry with paper towel. If necessary, slice each filet in half so it can fit in the pan. Liberally season both sides of each filet with the spice mix.
Preheat oven to 200F. Preheat a non-stick skillet over medium heat. Add olive oil and heat until shimmering.
Working in batches so as not to overcrowd the pan, fry the fish, undisturbed, until a dark brown crust forms, around 3 to 4 minutes. Flip and cook until a dark brown crust forms and the centre is opaque and flaky, around 3 to 4 minutes. Transfer the cooked fish to a platter and, while it's still hot, brush the top of each filet with one teaspoon of the compound butter. Keep the platter warm in the preheated oven while you repeat this process and cook the rest of the fish. Serve immediately.