Cut fillets into 3-4 ounce pieces, fold in half and season with salt and pepper.
Place in buttered flameproof baking dish. Add onion, parsley, lemon juice, wine and water.
Cover dish and heat on medium heat to a boil. Lower heat and simmer 6 to 8 minutes, or just until the fish flakes easily.
Transfer fillets into to flameproof serving platter with a slotted pancake turner and keep warm.
Stir Fish Veloute Sauce into liquid in baking dish. Cook until mixture is reduced one-third. Stir in butter until the butter is melted.
Strain sauce through a couple of layers of cheesecloth and spoon over the fillets. Place platter under a very hot broiler just until the sauce turns golden.
Make a pyramid of heated seedless grapes in the middle of the platter and serve immediately.
For Fish Veloute Sauce, melt the butter in a heavy saucepan. Stir in flour and cook very slowly until flour starts to change color and turn golden brown. Heat the clam broth (or fish stock/court bouillon), then add to the roux a cup at a time, mixing all the time with a wire whip. Skim when necessary. Add the remaining ingredients and cook very slowly for 1 hour, stirring often. Skim when necessary. When the sauce is the consistency of heavy cream, remove from heat. Strain through a fine sieve and cool. Stir occasionally while cooling to prevent a skin from forming on the top. Cover and store in refrigerator. Use within 1 week.