Make diagonal cuts in the skin of the duck breasts and season with salt and pepper. Fry, skin side down, in a hot frying pan for 2 minutes until the fat begins to run. Roast in the oven at 375 degrees F for 4 minutes. Remove, baste with the pan juices then roast for 2 more minutes and remove. Drain on kitchen paper to remove any excess fat. To serve, pour a little sauce onto each plate and top with the duck cut in thick slices. Garnish with the peach slices and peppers.
Peach and pepper sauce:
Pour off all the fat from the pan, add the wine and deglaze, scraping the pan with a spatula. Cook until reduced by one quarter. Add the stock, after first setting aside a few spoonfuls, and the butter. Dissolve the cornstarch in the remaining stock then slowly pour into the boiling mixture to bind the sauce, stirring all the time. The sauce should be slightly thick. Wash and pit the peaches. Cut the flesh into pieces and add. Also cut the peppers into pieces and add, then boil the mixture a little. Blend the sauce and strain well. Re-heat until a good color and serve.
Blanch the remaining peaches, peel, slice and bring to a boil in the sauce together with the Piquillo peppers.