Step 1Heat the oven to 400 degrees. Rinse, dry and season the fish with salt and pepper. With a sharp knife, score the fish, three or four times per side.
Step 2In an ovenproof saute pan (with a lid that fits) large enough to accommodate the fish, cook the onions, carrots and celery in the olive oil over low heat for about 10 minutes, until crisp-tender and fragrant. Add the thyme and garlic, and cook about 5 minutes more, until the vegetables are tender.
Step 3Place the fish on top of the vegetables, then add three-fourths cup wine. Bring the wine back to a simmer, then cover the pan and put it in the oven. Bake until the fish is cooked through, about 15 to 20 minutes.
Step 4Remove the pan from the oven and gently remove the whole fish to a platter, leaving the mirepoix in the pan. To make the sauce, add the remaining quarter-cup wine and place the pan on the stove top over medium-low heat. Whisk or swirl the butter into the mixture until the butter has melted. Add salt and pepper to taste.
Step 5Spoon the sauce around the fish. Sprinkle with fresh herbs. Serve whole or fillet at the table.