Step 1Whisk together miso, water, Sherry, sugar, ginger and onion. Place filets in shallow glass dish and coat with miso mixture. Cover and refrigerate several hours or overnight.
Step 2Scrape excess miso from fish to prevent burning. Cook on stove top grill pan sprayed with nonstick cooking spray over medium-high heat until fish flakes easily when tested with fork, 3 to 5 minutes each side. Brush filets with miso sauce during last few minutes of grilling to glaze. Serve with steamed brown rice sprinkled with ground seaweed.