Preheat oven to 350 degrees.
Slice tops of potatoes; scoop out pulp, reserving shells.
Into a mixer bowl place potato pulp.
In a skillet over medium heat melt margarine.
Saute until moisture evaporates.
Into potato shells spoon mushroom mixture.
To potato pulp add salmon and milk.
Beat until smooth.
Add onion and salt and pepper.
Spoon into potato shells.
On a baking sheet place stuffed potatoes.
Bake until heated through.
Garnish with fluted mushrooms.