One of my favorite things in the world! There are some regional variations, though. I think Italians have something similar called involtini. Germans cut their beasts up differently, so getting exactly what the traditional recipe uses is difficult in the US. Your best choice would probably a flank steak but that is still too thick. I made it with flank steak once that I cut in half horizontally, ending up with two very long and thin slabs of meat. If you buy them at a German butcher's, they are probably 1/4" x 6" x 15". Story on the side: I went to Reading Terminal Market in Philadelphia to an Amish butcher to get some meat and described to him that I needed a long, thin cut. 'Ah, you want to make rouladen!', he said. I had forgotten that they came over from The Dutch/German region long ago and kept their traditions.
Anyway, long, thin strip of beef. Salt, pepper, smear with a little bit of mustard. Add a layer of very thin strips of cured ham if you can find it. Prosciutto will work but the flavor is stronger than I like, very lean bacon should also work. Some use fnely diced onions also, I usually don't. Add a quartered pickle and roll everything up from the narrower side. You can bind them, use tooth picks etc. to make sure they don't open and fall apart. Brown them, in batches if you have many, very dark from all sides in a heavy pot for braising. I sometimes very lightly flour them before I brown them, my Mom doesn't. I also add 2 spoonfuls of sugar beet syrup to the sauce (get that in care packages from my folks in Germany), find it rounds it nicely, but my Mom doesn't. Getting them as dark as possible without burning them makes a fabulous sauce. Sautee some diced onions for a moment in the same pot, add water and/or beef stock and put everything together. Braise for 1 1/2h or so - not sure about the time, but my personal preference is rather longer than shorter. My Mom actually cooks them in a pressure cooker, but I never tried that and am even less sure about the cooking tome there, gotta ask her. I actually like the jus/sauce at least as much as I like the meat which will be a little stringy after long braising. You can adjust taste and bind the sauce a little bit with a flour slurry, but if I floured the rouladen before browning, I find the resulting sauce thick enough. Again, there are regional differences about what to serve with it. My preference are just boiled potatoes, but potato dumplings are also common. Braised red cabbage also goes great with them and is a very common side dish.
Hope this helps. Now I am really hungry, and I defnitely need to make them soon for myself. Btw, as so many braised dishes, they are at least as good the next day and they freeze well, so making a few extras is a good idea.
Stefan