I attended one of the classes at the Big Green Egg HQs a few weeks back with the 'Chef & the Fatman' that highlighted this great dish. Although Sea Bass is definitely not the cheapest cut of fish, this has to be one of the best things I have ever cooked on the Egg. I highly recommend it for a special occasion, or any night, if you have the cash to burn. I slightly altered my recipe from what the 'Chef & the Fatman', but not by much.
1/3 cup of Sake (can find at any Total Wine or larger liquor store)
1/3 cup of Rice Wine Vinegar (can find at any larger grocery store)
1/3 cup of 'Miso Mayonaisse' (they sell this at Whole Foods) - (you can use light yellow miso as well)
3 Tablespoons of (packed) brown sugar
2 Tablespoons of Soy sauce
2 Tablespoons of Fish sauce
4 Six Ounce Sea Bass filets (about 3/4 to 1" thick) (I got mine at Whole Foods, previously frozen, but thawed)
Chopped Green Onion and Fresh Basil for garnish/finish
1 Alder wood grilling plank, soaked for 1-2 hours in cold water
Mix first six ingredients together and pour over fish and then refrigerate. Let marinade for 2-6 hours. I let mine marinade for closer to six hours. Heat up BGE to 400-450. As you can see, I did mine with the Tablesetter legs down and then the pizza stone propped up with planter's feet to let air circulate underneath. This is also how I cook my pizza, but at a much higher heat. Cook fish about 15-20 minutes, however watch closely depending on thickness. You want the fish at about 130-140 degrees. 130 is how they serve it in restaurants. Sprinkle the green onion and basil over the fish and enjoy. I recommend a mushroom risotto or something of the sorts as a side.