Place each strip of beef between 2 sheets of plastic wrap and pound until very thin. You want a maximum thickness of 1/4".
Sprinkle each beef strip with salt and pepper.
Spread approximately 1-2 tablespoons of mustard on each beef strip. Be generous here. Don't be afraid of adding too much.
Lay 1-2 strips of bacon on each beef strip. 1 for smaller strips and 2 for larger ones.
Sprinkle each beef strip with the thinly sliced onion. Again be generous.
Starting from the narrowest end of the strip, start rolling the beef up. As you're rolling start tucking each side in a little so that the edges are sealed and the filling is contained. Once finished, tie the roll with cooking twine or secure with a toothpick at each end. Don't worry if it doesn't look pretty and things stick out a bit. It will still be good. Continue until all the Rouladens are rolled.
In a large frying pan or dutch oven, heat up a couple of tablespoons of olive oil and brown all sides of the Rouladen until they're a deep brown color. This fond will help give the gravy flavor.
Once the Rouladen is well browned, add the beef stock and water. You want just enough that the liquid comes about 3/4 of the way up the Rouladen.
Cook over medium-high heat until the broth comes to a boil. Then reduce the heat to low, cover the pot and continue to simmer for another 1 1/2 to 2 hours or until the Rouladen are very fork tender.
During this time check the pot occasionally to make sure there is still enough liquid. If not, add more broth and water in equal amounts.
Once the Rouladen are fork tender remove them from the pan and reserve on a plate and cover to keep warm. Turn the heat to medium-high and reduce the liquid by about 1/3rd. At this point, you can add a slurry mixture to make a gravy or leave as an au jus as I like to do.
Once the liquid is reduced or you've finished the gravy check the seasoning and add salt and pepper if needed. Add the Rouladen back to the pan to stay warm while you make the Spaetzle.