Chop the shallot and fresh thyme. Set aside.
Score the duck breast's fat side several times, slicing with a knife about 2 centimetres deep almost to the meat under the fat. This allows the meat to cook more evenly.
Heat a heavy-bottomed saucepan for one minute on high heat. Then place the duck breast fat side down to sear until it is browned.
Remove the duck and set aside. You can now sautée the mushrooms directly in the duck fat - should be a thin layer of fat, or you can drain away excess, use olive oil or butter if you prefer.
Sear the sliced mushrooms to a golden brown. If you are using dried porcini, you first need to soak them in liquid, a hot broth like a vegetable stock is best, for 10 minutes to bring them back to life - then pat them dry with paper towels before searing.
After cooking set the mushrooms aside.
Reduce the heat so that the pan doesn't get too hot and begin to burn.
Resume searing the other side of the duck breast using this same pan - using high heat as you want to sear the outside to a golden brown.
After searing the duck meat side down to a nice brown colour, turn the heat down and add the shallots. After cooking the shallots for around 1 minute or until they are translucent but only slightly brown, deglaze the pan with the port wine, leaving the duck breast in the pan. Stir and scrape the saucepan's bottom to collect all the flavours from the sautéed mushrooms, shallots and duck.
Let this reduce and further cook the duck to your desired temperature - 2 - 3 minutes for rare.
Remove the duck breast and let it sit at room temperature for a good 5 minutes before slicing.
Allow the port sauce to continue to simmer and reduce in the pan, adding salt, pepper and thyme. Turn off the heat and set aside.
Slice the duck breast into thin portions and plate 3-4 slices per person.
Serve 2 tablespoons of the sauce from the pan over your duck breast, then scatter the sautéed mushrooms on top or on the side.