1. Bring 4 cups water to a boil in a saucepan over medium-high heat. Add potatoes, and cook 15 minutes. (Potatoes will not be fully cooked.) Drain and set aside.
2. Preheat oven to 325°. Heat oil in a large ovenproof skillet over medium heat. Add onion, and sauté 3 to 4 minutes or until soft and slightly brown. Add garlic and tomato, and sauté 2 minutes more.
3. Place fish in pan, and add reserved parboiled potatoes, thyme, and next 4 ingredients. Bake, covered, at 325° for 22 to 25 minutes or until fish flakes easily. Remove from oven, add butter, re-cover, and let stand 3 minutes or until butter melts. Serve warm.
*Substitute other thick white fish, such as haddock,if desired.