I made this a few nights ago and wanted to share the "recipe" even though it wasn't a recipe.
We had 2 pounds of rouladen-cut beef in the freezer. Rouladen, if you've never had it, is beef pounded thin, rolled around onions or pickles, and baked with gravy. At the local supermarket they sell the beef already pressed--saves some work. But I decided to use the meat for a crockpot meal instead.
So: in the crockpot went one onion, sliced thin; 2 pounds rouladen-sliced beef, cut up (but I think stew-meat cubes would work as well); a quarter cup of Diana Sauce (I limited the amount because of sodium concerns, but you might like more), and several large mushrooms on top. I started it on high because the meat was a bit frozen, but turned it down to low later on. Later I added a bit of flour mixed with liquid, to thicken the gravy.
And that was it! The sauce, mushrooms and beef combined to make a really rich, dark steak-and-mushrooms gravy, and we had it all over mashed potatoes.