Preheat the oven to 425.
First, add potatoes, salt, and chicken stock to a pot.
Now turn the heat to medium high.
When the stock boils, cook for 15 minutes and drain.
Return the potatoes to the pot and add mustard, black pepper, and cream.
Now mash until creamy consistency.
Next add the 1 cup of gruyere, half the scallions, and mix.
Add mashed potatoes to a buttered baking dish.
(NOTE: If you want to make ahead, stop here. Cover and put in the refridge until ready to use.)
Next top the potatoes with the remaining cheese. Put on top rack in oven, and cook for about 5 minutes or until the cheese browns slightly.
Finally top with bacon and remaining scallions and enjoy!