Rinse fish thoroughly in cold water; pat dry. Rub fish inside and out with 1 teaspoon salt and pepper. Cut 3 slits in top side of fish; stuff each opening with 1 slice bacon. Set aside.
Place potatoes, onion, and remaining salt in a small Dutch oven. Add water; cover and steam until tender. Drain vegetables, reserving 1/2 cup cooking liquid; set vegetables aside, and keep liquid warm.
Place fish in a well-greased 13- x 9- x 2-inch baking dish. Bake, uncovered, at 400° for 10 minutes or until fish is lightly browned. Remove fish from oven; arrange potatoes and onions over top. Pour tomato sauce and reserved liquid over vegetables. Return to 400° oven, and bake 20 minutes or until fish flakes easily when tested with a fork, basting once with pan drippings.
Garnish with lemon wedges and parsley sprigs; serve immediately.