Place beef roast in crock pot or roasting pan that is slightly larger than roast. Spread cream of mushroon soup on top of roast only. Pour brown onion gravy mix on top of soup only. Gently pour pepsi or diet pepsi around sides of roast only, until pepsi is about 1/2 way up sides of roast.
Place lid on crock pot and cook for 5 to 6 hours. If baking, place lid on roaster and cook in a slow oven (about 300 degrees) for about 4 hours.
Allow roast to rest for about 5 minutes and then serve. Great reheated.