In your slow cooker, add the soup, beef stock, and dry onion soup mix, whisk to mix, lumps are ok, they will work out when it cooks.
Cut beef into about 1 1/2 inch cubes. Sprinkle beef with ground black pepper or lemon pepper if you have that. Add the beef to the slow cooker and stir to get everything mixed.
Cover and cook on low for at least 6-8 hours. Check at 6 hours, beef should be tender.
Now, you can serve this up just as it is when it is ready. But if a thicker gravy is desired it couldn't be any easier to make. First, remove the beef from the slow cooker, draining the liquid into the slow cooker. Melt the butter in a small sauce pan, then add 3 heaping tablespoons of flour, whisk together until it turns a light golden color, then add 2 cups of the liquid from the slow cooker. Whisk this until smooth, whisking continuously, then increase heat and it will start to thicken, when you have the right consistency, remove from the heat and pour into the slow cooker, mix, then add the beef and mix to coat the beef.
Serve over rice, mashed or boiled potatoes, or egg noodles. A vegetable on the side makes this a complete meal.