Rinse the grapes, pluck them, cut them in half and remove the seeds. Remove the skin from the onion and dice very finely. Rinse the fish fillets and dab with kitchen roll, divide into 4 portions, sprinkle lightly with salt and pepper on both sides.
2. put the wine in the saucepan with one-eighth of a liter of water, mustard and peppercorns and bring to a boil. Lightly brush the strainer insert with oil to prevent the fish from sticking. Pour in fish pieces as close together as possible and cook over steam at a good medium heat (6-8 min each depending on thickness). Keep the fish warm with the lid closed.
Heat the butter in a frying pan and sauté the onion until translucent. Fold in grapes and one-eighth of a quart of the strained liquid from the steamer and make 1/2 minute over good medium heat. Fold in whipped cream and mustard. Season all well with salt and freshly ground pepper and simmer over low heat until sauce is slightly thick.
Rinse sorrel, dry leaves between paper towels, then cut into strips. Cut the peppers in half lengthwise, remove the seeds and the stalks. Rinse the pod halves and cut into narrow strips. Mix half of sorrel into sauce just before serving.
Arrange fish on hot plates and bring to table sprinkled with remaining sorrel and pepperoni strips.
Serve with fresh basmati or bague.