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Mix together 2 teaspoons (11 g) of sea salt, 1 teaspoon (2 g) of fresh cracked pepper, 1 1/2 teaspoons (3 g) of paprika, 1 tablespoon (6 g) of dried thyme, and 1 teaspoon (2 g) of cayenne. Lay 1 to 2 pieces of opah on a plate and brush 1 to 2 tablespoons (15 to 30 mL) of melted butter or olive oil over the fish. Spread the blackening seasoning evenly over the fish. Use your fingers to rub the seasoning into the fish and set it aside while you heat the grill. Turn a gas grill to high or heat a full chimney of briquettes. Once the briquettes are hot and lightly covered in ash, dump them in the center of the grill grate. Brush a little canola or vegetable oil on the grate to prevent the fish from sticking to it. Lay the seasoned opah on the grill so it's directly over the briquettes or the hottest part of the grill. Cook the opah for 5 minutes so it blackens on the bottom. Use tongs to flip the fish over and grill it until it's cooked on the outside. The opah will be medium-rare after 3 to 5 minutes, but you could cook it 2 to 3 more minutes, if you prefer your fish well-done. If the fish is medium-rare, it will be soft and tender. Well-done fish will be completely dry, firm, and flaky. Remove the opah from the grill and serve it immediately. Blackened opah goes well with steamed rice, cabbage, or roasted vegetables. Refrigerate leftover blackened opah in an airtight container for 3 to 4 days.

Summary:
Combine the blackening seasoning in a small bowl. Brush the opah with butter or oil and rub the blackening seasoning over it. Heat a gas or charcoal grill. Grill the opah for 5 minutes. Flip and grill the opah for 3 to 5 more minutes. Serve the blackened opah.