Thaw fish, if frozen; set aside. Cut kernels off fresh corn; set aside. In a large saucepan, combine cobs, vegetable stock and 2 sprigs thyme. Bring to a boiling; reduce heat. Simmer, uncovered, for 20 minutes. (Add a little water if the liquid goes below about 1 cup.)
Meanwhile, wash potatoes; pat dry with paper towels. Cut crosswise into 1/4-inch thick slices; set aside. Strain stock mixture; reserving cobs. When cobs are cool enough to handle, hold cob over a bowl and scrape with a sharp knife to release all the juices; discard cobs. Stir juices into stock (should have about 1-1/2 cups stock); set stock aside.
In the same large saucepan, cook and stir potatoes, corn kernels, onion and celery over medium heat in butter for 5 minutes. Add the stock mixture, cream, thyme, 1 teaspoon parsley, 1/2 teaspoon salt and 1/4 teaspoon black pepper. Bring to boiling; reduce heat. Simmer, uncovered, about 15 minutes or until the vegetables are tender.
Meanwhile, rinse fish; pat dry with paper towels. In a 12-inch skillet, heat oil over medium-high heat. Season fish lightly with salt and pepper and add to skillet in a single layer. Tuck under any thin edges. Cook fish for 4 to 6 minutes per 1/2 inch of thickness or until fish flakes easily when tested with a fork, turning once.
Serve fish with chowder or atop chowder; sprinkle with additional parsley, if you like. Serve immediately. Makes 6 servings.