Heat the butter and oil in a heavy Dutch oven or or large deep skillet with a lid over medium to medium high heat.
Sear the rouladen in batches, browning well on all sides. Do not crowd rouladen in skillet, or they will not brown nicely. Add extra butter and oil if needed. Adjust the heat if the meat is scorching.
Add the onions to the pot and a little more butter or oil if needed. Cook the onions over medium heat until softened and translucent, about 5 minutes.
Stovetop: Bring to a boil and then reduce and cover. You can cook the rouladen, covered, on the stovetop on low for about 90 minutes or until fork tender. Turn the rolls, moving ones from the bottom to the top so that they all cook evenly.
This is enough to feed eight people. Feel free to double the recipe so that you will have enough to freeze or for leftovers.
A butcher is your friend for getting the right meat cut properly. Depending on the cut, the size may be smaller or larger than stated. If they are on the larger side, then one per person is good. If smaller, then be prepared to make two per person.
I don't like a whole slice of bacon, I feel like it can get a little greasy. But if the bacon you are using is on the thinner or smaller side, use full rashers per roll.
Feel free to use toothpicks if you would rather, for securing the rolls. I don't like using toothpicks.
You can use beef stock, beef bouillon powder or paste, or even just water.
To use the slow cooker, after cooking the onions through wine, put everything into the slow cooker and cook on high 3-4 hours or low 6-8 hours.
You can prepare the rouladen the day before up to the point of making the gravy. Store everything in an airtight container in the fridge. The next day re-heat over medium low heat, either in the oven or stove top until the meat is warmed through. Then remove the rouladen and make the gravy as described above.
To freeze: Take the cooled rouladen and liquid and place into freezer safe container(s) covered with the cooking liquid. Thaw, then warm through as described above, and then make the gravy. Or if the gravy is already made, then cover the cooled rouladen with the gravy and freeze.