Clean out the skillet. Butter 1 side of 4 slices of bread and place butter side down in the skillet. Spread a portion on deli mustard on the top side of this bread. Lay down a slice of cheddar cheese; divide the ham amongst the 4 sandwiches, top the ham with the mushroom and onion mixture. Top each with a slice of Swiss cheese. On 1 side of the last 4 slices of bread, spread a portion of horseradish sauce. Place on sandwiches. Lastly, spread butter on the top side of each sandwich.
Brown the sandwiches on each side until golden brown.
Recipe by simple-food-solutions.com at http://simple-food-solutions.com/?p=2118