Place the beef in the slow cooker, then add the consumme, beef broth, worcestershire sauce, onion flakes, and minced garlic. Close the lid, set the cooker to high, and cook 2 hours.
After 2 hours reduce the cooker to low and cook for approximalty 5 hours.
In a smal bowl, mix the mayonaise and horseradish. Set aside.
When the beef is almost ready, sautee the mushrooms in a skillet with a bit of butter or oil until soft.
Pre-heat oven to 400 degrees
Remove the beef from the pot and place it in bowl. Pull the meat apart with 2 forks until it is shredded.
Cut each bun open and lay them on a baking sheet. Spread some of the horseradish sauce on each side of the buns. Then place a large pile of beef on one side of each bun.
Top the beef with a scoop of mushrooms, then a handful of cheese.
Place the tray in the oven and bake for about 5-7 minutes until cheese is melted and bubbly.
Remove the tray from the oven and close each sandwich.
Whisk the remaining juices in the slow cooker to help emmusify any seperation, then ladle it in to small bowls.
Cut each sandwich in half and serve with a bowl of the jus for dipping.