Preparation Heat 1 tablespoon oil in heavy large saucepan over medium-high heat. Add onion, celery, carrot and garlic; sauté until brown, about 10 minutes. Add tomato paste and cook 2 minutes. Add white wine and simmer until liquid is reduced by half, scraping up browned bits, about 5 minutes. Add next 7 ingredients; simmer until mixture is reduced to 3 cups, about 50 minutes. Discard bay leaf. Season sauce with salt and pepper. Preheat oven to 350°F. Sprinkle halibut with salt, pepper and cayenne pepper. Heat remaining 1 tablespoon oil in heavy large skillet over high heat. Add fish to skillet and cook until brown, about 2 minutes per side. Transfer fish to plate. Pour off oil from skillet. Reduce heat to low. Add balsamic vinegar to skillet; simmer 1 minute. Add sauce. Whisk in butter. Season with salt and pepper. Pour sauce into 13x9x2-inch glass baking dish. Place fish atop sauce in dish. Bake until fish is opaque in center, about 10 minutes. Divide potatoes among 6 plates. Top potatoes with fish and sauce. Sprinkle with Gremolata and serve immediately. *Available at Asian markets and in the produce section of some supermarkets.
Preparation Heat 1 tablespoon oil in heavy large saucepan over medium-high heat. Add onion, celery, carrot and garlic; sauté until brown, about 10 minutes. Add tomato paste and cook 2 minutes. Add white wine and simmer until liquid is reduced by half, scraping up browned bits, about 5 minutes. Add next 7 ingredients; simmer until mixture is reduced to 3 cups, about 50 minutes. Discard bay leaf. Season sauce with salt and pepper. Preheat oven to 350°F. Sprinkle halibut with salt, pepper and cayenne pepper. Heat remaining 1 tablespoon oil in heavy large skillet over high heat. Add fish to skillet and cook until brown, about 2 minutes per side. Transfer fish to plate. Pour off oil from skillet. Reduce heat to low. Add balsamic vinegar to skillet; simmer 1 minute. Add sauce. Whisk in butter. Season with salt and pepper. Pour sauce into 13x9x2-inch glass baking dish. Place fish atop sauce in dish. Bake until fish is opaque in center, about 10 minutes. Divide potatoes among 6 plates. Top potatoes with fish and sauce. Sprinkle with Gremolata and serve immediately. *Available at Asian markets and in the produce section of some supermarkets.