Salt the salmon and allow to rest in refrigerato 1 hour. Boil potatoes and carrot until just soft. Combine potatoes, carrots, onon and pepper. Butter 2 squares of foil. Divide the poato mixture in half, place in center of each foil square. Set filet atop misture, press down to keep in place. Top each filet with a pat butter, a slice of lemon, and some mushrooms. Fold foil tightly, bake at 400* for 15 minutes.