Preheat the oven to 400 F. Place the cubed potatoes into a large saucepan and cover with water. Cook over medium heat until fork tender. Drain potatoes, and place into a large bowl.
Mash the potatoes with the butter, salt, pepper, and 1/2 cup shredded pepper jack cheese until smooth using either a potato masher or mixer. Spread the potatoes out into a greased round baking dish.
Top with the remaining pepper jack cheese and fried onions. Bake for 15-20 minutes.