Assemble the rouladen: Slice the dill pickles in half lengthwise, then slice each half again lengthwise so you have 12 small spears. Lay a slice of beef on a cutting board. Season all over with a pinch of salt and pepper. Add about half a tablespoon of the mustard and smear it all over the beef. Lay a piece of bacon lengthwise onto the beef, then add 2 pickle spears and a few slices of onion. Tuck the long sides of the rouladen into the center about ½ inch, then roll up from short side to short side. This ensures the fillings will stay inside the rouladen. Secure with toothpicks or butcher’s twine. Repeat for all of the rouaden, then season the outsides with more salt and pepper.

Sear the rouladen: Set a Dutch oven over medium-high heat and add the vegetable oil. Once hot, add the rouladen (depending on the size of your Dutch oven, you may have to work in batches so you don’t crowd the pot) and sear on all sides until the beef is dark brown all over, about 5 minutes per side. Transfer them to a separate plate and turn the heat down to medium.

Prepare the pan sauce: Add the onion and carrots and let cook for about 5 minutes, until softened. Add the tomato paste, stirring well to combine, and let cook until the tomato paste turns dark and starts to stick to the bottom of the pot, about 3 minutes. Sprinkle in the flour and stir, letting it cook a little bit too. Once the flour is incorporated, add the red wine and use your spoon to scrape up all the stuck-on bits. Add the beef stock and bring everything up to a simmer.

Braise: Once the sauce in the pot is simmering, add the rouladen back to the pot, tucking them into the sauce. Cover and reduce the heat to low. Let braise for at least 1 hour. Then, check them by sticking a sharp paring knife into the center of one and lifting it gently up. It should be so tender that it falls straight off the knife right away, barely making it out of the pot. If it’s not that tender, keep braising, covered, until it is.

Reduce the sauce: Once the rouladen are tender, remove them with tongs to a serving platter and remove the toothpicks or butcher’s twine. Let the pan sauce reduce over medium-high heat until thick, about 10 minutes. Remove from the heat and add butter and a splash of pickle brine, if you'd like. Then taste and season with salt and pepper as needed.

Serve: Pour the gravy all over the rouladen and serve immediately with your choice of sides.