Preheat oven to 350F.
In a pan on medium heat, add some butter along with your chopped mushrooms, sauteing for about 5 minutes until they begin to soften.
Slice your potato in half, and using a spoon, remove the inside to a bowl.
Add roast beef, blue cheese, mushrooms, and green onions to the bowl, mixing together and adding salt & pepper to taste.
Place filling back inside potato skins.
Bake at 350F for ten minutes, then turn to a broil, cooking until the top is crisped.