Add the prepared potatoes to a large stockpot and cover with cold water.
Cover the pot with a lid.
Bring the water to a low-boil and add 2 tablespoons salt. *Note - keeping a wooden spoon in the pot will stop any boilovers.
Reduce the heat slightly and simmer the potatoes until fork-tender (approximately 30 minutes).
Drain the potatoes and add them back into the hot pot.
Add the warm milk, room-temperature butter, and cream cheese into the potatoes and mash.
*Note - if ricing the potatoes, after draining, rice them into the hot pot and then add the remaining ingredients to the riced potatoes and stir.
Add the horseradish, shredded cheese, black pepper, additional salt (if desired, to taste) and grated nutmeg.
Mash again until you reach your desired consistency.
Add the cheesy horseradish mashed potatoes into a 13x9 casserole dish which has been sprayed with baking spray.
To refrigerate: Cool Completely. Cover with foil or plastic wrap and refrigerate for up to 2 days.
To freeze: Cool Completely. Cover with foil or plastic wrap and freeze for up to 2 months.
If serving immediately, place in a preheated 350-degree oven for 20 minutes, uncovered.