All you need: Deli roast beef, favorite cheese (I use provolone), hoagie buns, butter, au jus packet and 3 cups of water.
I preheat my oven to 350. Butter both sides of the hoagie bun. Then pile on roast beef (I put 3 slices per sandwich). Pop it in the oven for 10-15 minutes so the bread is crusty and the meat is warmed through.
In the mean time, put au jus packet contents into a sauce pan and whisk in 3 cups of milk. Boil over medium-high heat.
When sandwiches are warmed and crusty, slap on a couple slices of cheese and pop back in the oven until cheesy is gooey and melty.