Lay out the beef slices on a work surface. Pound them gently with a meat mallet to flatten them further if necessary. Season both sides with salt and pepper.

Spread a teaspoon of mustard on each slice. Place a slice of bacon, some slices of onion, and a pickle on one end of each beef slice.

Roll up the beef slices tightly around the filling, starting from the end with the filling. Secure each roll with toothpicks or kitchen twine.

In a heavy-bottomed pan or Dutch oven, heat the vegetable oil over medium-high heat. Brown the rouladen on all sides, then remove them from the pan and set aside.

In the same pan, add a bit more oil if needed. Stir in the flour to make a roux, cooking for a minute or two. Gradually whisk in the beef broth and red wine, if using. Add the tomato paste and bay leaf. Bring to a simmer, stirring constantly until the gravy thickens. Season with salt and pepper.

Return the rouladen to the pan, spooning some of the gravy over them. Cover and simmer on low heat for about 1 to 1.5 hours, or until the meat is tender.

Carefully remove the toothpicks or twine from the rouladen. Serve them with the gravy and your choice of side dishes.

Meat Selection: Ask your butcher for beef top round slices specifically for rouladen, or buy a top round and slice it thinly at home.

Cooking Time: Adjust the braising time based on the thickness of the beef slices. The goal is for the beef to become very tender.

Thickening Gravy: If the gravy is too thin after cooking, you can remove the rouladen and bring the gravy to a boil. Mix a little cornstarch with water and whisk it into the boiling gravy to thicken it.

Serving Suggestions: Traditional sides include mashed potatoes, spaetzle, or boiled potatoes, and red cabbage or green beans.