Lay the beef slices flat on a cutting board. Spread a thin layer of mustard on each slice.
Place a slice of bacon on each beef slice, followed by some chopped onion and minced garlic. Place pickle slices along the length of each beef slice.
Roll up each beef slice tightly and secure with toothpicks or kitchen twine. Season the outside of the rouladen with salt and black pepper.
Heat the vegetable oil in a large skillet or Dutch oven over medium-high heat.
Brown the rouladen on all sides, about 2-3 minutes per side. Remove from the skillet and set aside.
In the same skillet, add the remaining chopped onion and cook until softened and golden brown.
Stir in the flour and cook for 1-2 minutes until lightly browned.
Gradually add the beef broth and red wine (if using), stirring to combine and create a smooth sauce.
Return the browned rouladen to the skillet, ensuring they are partially submerged in the sauce.
Bring the sauce to a simmer. Cover the skillet and cook the rouladen for about 1.5 to 2 hours, or until the beef is tender. Turn occasionally to ensure even cooking.
Once the rouladen are cooked, remove them from the skillet and let them rest.
Stir the butter into the sauce to add richness. Adjust seasoning with salt and pepper to taste.
Remove the toothpicks or twine from the rouladen. Slice them if desired.
Enjoy your meal!