Amber's French Dip Sandwiches

1 large cross rib roast (Watch for the sales. I buy a cross rib roast on sale for just over $6.00.)
2 packages au jus mix
1 sliced onion (optional)
Water

1. Place rib roast in crock pot.
2. Pour water over roast, so that the water line is a little more than ¼ full.
3. Top with onion and two packages of au jus mix.
4. Cook on low for 8 hours. At least one hour before serving, shred roast with a fork. This is also a good time to trim fat from the roast. Cook on high for remainder of cook time.
5. Serve with your choice of rolls and a cup of the au jus from the crock pot.