Taken From http://crockpot365.blogspot.com/
5 lbs bone-in rib eye (this is the max that will fit in a 6.5 qt crockpot)
1 large yellow onion, sliced in rings
6 cloves garlic, peeled
2 T cornstarch
1 tsp kosher salt
1 tsp black pepper
1 T Worcestershire sauce
1/2 cup beef stock/broth
Slice the onion, and spread around the bottom of your crockpot. Trim excess fat off of meat. Make little slits in the meat with a serrated knife, and insert a garlic clove into each one. Try to space them out equally, if you can.
Add dry ingredients to a plastic zipper bag, and shake and bake the meat. Put it in the crockpot on top of the onion slices. Add Worcestershire sauce and broth. Cover and cook on high for 4 hours, on low for 6-8, or until meat has reached desired temperature with a meat thermometer.
Let the meat stand for 15 minutes before slicing. If you'd like, you can use the juice at the bottom of the crockpot to make a gravy with some more cornstarch. Do not put your crockpot on the stove. Pour the drippings into a pot.