Preheat oven to 375 degrees. Lightly grease a 9 by 9 inch baking dish.
Melt 4 tablespoons of the butter and saute the onion and celery with the thyme until soft about 8 minutes. Add the sherry and cook for 2 minutes longer. Add the bread crumbs, remove from heat and mix well. Season with salt and pepper to taste.
Place half of the fillets in the baking dish. Cover with half of the stuffing. Repeat with the rest of the fish and the stuffing. Top with the tomatoes making sure that they are broken into small chunks.
Pour the juice from the can around the sides of the baking dish. Drizzle the remaining butter over the top.
Bake the haddock for 35 to 40 minutes or until the fish easily flakes when tested with a fork. Serve at once.
