Lay each piece of salmon on a sheet of plastic wrap, about 1 0 by 16 inches. Fold sheet in half and pound flat with a mallet or saute pan. Open sheet and lay carpaccio on individual salad plates. Wrap tightly and keep in the refrigerator. In a saute pan coated lightly with olive oil, saute the shallots for 1 minute then add the maitakes. Season and cook for 1 minute. Meanwhile, whisk together the ponzu, Dijon and olive oil. Season and check for flavor. Pull the pan off of the heat and add basil and a tablespoon of the vinaigrette. Check for seasoning.
PLATING: Remove salmon from the refrigerator 5 minutes before serving and season with fleur de sel and black pepper. Place warm maitakes on top of the salmon and drizzle with vinaigrette.