Preheat oven to 400ºF.
On a baking sheet, roast the potatoes until cooked through, about 45 minutes. When cool enough to handle, cut a quarter of an inch slice off the top of the potato to create a large boat. Scoop out all the potato flesh into a medium bowl, keeping the skin intact.
Mash the potato flesh with the spicy brown mustard, milk, butter, salt, ground black pepper and chives. Fold in the chopped corned beef.
Fill the potatoes with the corned beef-potato mixture so they are over-stuffed. Combine the cabbage with the vinegar (or you can use sauerkraut). Pile cabbage or sauerkraut over the potatoes then top with shredded Swiss cheese. Pop into the oven until golden and bubbly on top.