To temper the fish before cooking it simply means allowing it to come to room temperature. If the fish goes into the pan cold, it will steam, rather than sear.
Remove the fish 15 minutes before cooking to "temper." Thoroughly pat dry each fillet on both sides with paper towel. Score the skin lightly with a sharp knife, carefully. Do not pierce the flesh.
Cut the orange and grapefruit sections into thirds. Place in a bowl with the celery and toss with the mint and herbs.
Heat a 12-inch nonstick skillet over very high heat. Just before searing, season the flesh side of the fish with salt and pepper. Drizzle in the oil and add the fillets, flesh side down to the pan. Sear for 2 minutes. Season the skin with salt and pepper and flip. Cook another 2 minutes. Both sides should be golden brown and slightly crispy. Serve each fillet with a couple tablespoons of citrus relish on top. Garnish with fresh mint.