Soften 4 tsp butter to spread. Spread on each bun to toast. Do not toast yet.
Put the other 4 tsp in a pan to saute your mushroom, onion, and pepper in. Start sauteing once the butter has melted and pan is hot. Make sure you cook this long enough for all the moisture to cook out of the pan. I prefer to cook each veggie separately that way if some one does not like one or the other they don't have to have it.
While the veggies are cooking. Start mixing, then warming your aux jux to a soft boil. Time this so that your veggies are just finishing cooking when your aux jux is starting to boil.
Remove your veggies, Put in a bowl and place the frying pan back on the burner to keep hot (this is to toast the buns till golden brown right before assembling).
In your soft boiling aux jux place your slices of roast beef just long enough to get hot and remove. DO NOT over cook your roast beef in the aux jus as it will make it very tough. Use your corn starch and water mixture to thicken the aux jux. Once the aux jux is thick as you desire, remove from heat and set aside. Then toast your buns.
Place a half a bun on each plate and load up your sandwich with beef and veggies. Scoop as much aux jux gravy over your bun as you like and place the other bun on top.
Eat with finger is you dare, this is pretty messy. I prefer a fork.