Preheat oven to 350°F.
In a bowl, combine the prepared potatoes, 1 cup of the cheese and 3 tablespoons of the chives.
Remove the stems from the mushroom caps. Place them cavity side up in a baking dish.
Place the potato mixture in a piping bag with a star or your desired tip. (It will need to be a tip with a large opening.) Pipe the potato mixture into the mushrooms. Drizzle lightly with olive oil.
Bake for 15 minutes, then top the mushrooms with the remaining ½ cup cheese. Return to the oven and cook until the cheese has melted and the mushrooms are cooked, about 10 more minutes.
Allow to cool for a couple minutes, then sprinkle with the remaining 1 tablespoon chives. Serve warm.
