Preheat oven to 400°
Clean the potatoes with a good scrub.
Pierce potatoes with a fork.
Bake at 400° for 1 hour or until tender.
Cut a lengthwise slice off of the top of the baked potatoes to create a bowl, but do not cut it in half.
Scoop out flesh, leaving about a 1/4-inch-thick shell.
In a medium-sized mixing bowl, combine flesh, sour cream, butter, cream/mascarpone cheese, fontina cheese, garlic and onion salts.
Mash until chunky, yet well-blended.
Divide mixture evenly among hollowed out potato shells.
Place potatoes in a single layer on a baking sheet.
Sprinkle potatoes evenly with Cheddar cheese.
Bake at 400° for 12 minutes or until heated.
If desired, sprinkle freshly-grated Parmigiano-Reggiano on top of each potato.
Preheat broiler.
Broil potatoes for 2 minutes or until browned and bubbly.
Garnish with pancetta/bacon, chives, and paprika.