I put it in the crock pot on low most of the day, just to let the juices simmer and soak into the meat etc. But you could really just throw it on the stove and warm it up for a few minutes.
Easy French Dip (Ah Jus Sauce)
1 box beef broth
1 lb roast beef sliced/shaved thin
1/2 tsp garlic powder
1/2 tsp onion powder
Put first 4 ingredients into a crock pot on low for 4-8 hours. With a fork, place roast beef onto a sub roll. Put reserved juice into individual bowls. Dip sandwich into the juice. Enjoy