Pre-heat the oven to 350 °F.
Caramelize the onions and fry the King Oyster mushrooms in butter.
Cut the hoagie buns in half and put face-up on a baking sheet.
Bake for 4 minutes, until the buns start to toast.
Add 2 slices of cheese to each bottom half of a bun. Bake for another 5 minutes, until the cheese just starts to melt.
Top each bun with meat, onions, and mushrooms.
For each bun, fill a small bowl with the liquid from the slow cooker.
Serve the French beef dip sandwich with the au jus on the side.