Place mashed potatoes in a pastry bag or a large heavy freezer bag. Squish the mashed potatoes to one corner of the bag and slice the tip of the bag off with scissors. Line the edge of 4 small oven proof dishes with a heavy stream of mashed potatoes, approximately 1/2 inch thick.
In a med size stove-top pot, melt butter at med-low heat. Add shallot and mushrooms to pan. Sauté for approx 2 minutes.
Sprinkle flour over this. Gradually whisk in half and half and then milk and sherry Stir until mixture begins to thicken slightly. Stir in salt, scallops and shrimps. Heat through.
Fill the center of each potato piped dish with the seafood sauce until it comes just past the bottom ridge of the piped potato. Grate a little gruyere cheese directly on top. Place in pre-heated oven. Set timer for 15 minutes, or until cheese starts to crust and potatoes form a golden crust.