Prick the skins of the potatoes, but leave them on and microwave until just tender, about 8 to 10 minutes, depending on their size. Let cool, then slice.
In a saucepan, combine the milk, sour cream, and cheese. Season with mustard, salt, nutmeg, and pepper. Without letting the mixture boil, cook over medium heat to melt the cheese. It may still look lumpy, this is okay.
Preheat oven to 350 degrees F and grease a 9x13-inch casserole dish. Add the sliced potatoes and sprinkle with the salmon and green onions. Pour on the sauce and bake, uncovered, for one hour.