Cut diagional slits through the skin and fat, but not into the meat. Into the slits, place hydrated green peppercorns, then sprinkle lightly with salt and pepper. If you buy the dry ones, hydrate them by soaking them in Cognac. Preheat the oven to 150 degerees. Take a heavy cast iron skillet and add a very tiny bit of peanut oil. You're not going to fry these; you're going to pan sear them and the tiny bit of oil is to help prevent sticking. Heat the the pan over high heat til the oil just begins to smoke then add the breast skin side down and cover and sear for three minutes, then turn and sear the meat side for three minutes, then turn once more and sear for two minutes. If they're thin, use 1 minute for the last period instead of 2.
Remove the magret and place on a platter. Cover with aluminum foil and place in the oven to keep warm. Add 2 tablespoons of butter to the pan, then 2 minced shallots. Stir with a flat-bottomed spatula. Don't let the shallots brown. Over high heat, it'll just take a minute or two. Then add maybe 2 or 3 ounces of Cognac and either flame or boil the alcohol off, stirring all the while to remove the seared particles from the pan bottom. Then add 3 ounces of dry Sherry. Reduce the liquid by about half and then add 4 ounces of heavy cream and 2 ounces of a good cream, Oloroso, or Amontillado Sherry and continue heating over high heat, adding the liquid accumulating on the duck platter to the pan. When the sauce begins to thicken watch carefully. The bubbles in the boiling sauce will begin to get larger. Remove it from the heat immediately or the sauce will separate. Stir in 1 tablespoon of a good Dijon mustard and spoon over the magrets. The duck should be medium rare. If not, adjust your time the next time you make this, and you will make this again.