Lay the beef slices out on a work surface. Spread each beef slice with about 2 teaspoons of mustard and sprinkle with a little salt and freshly ground black pepper.

Place a strip of bacon on each beef slice so it's running the same length as the beef. Place the sliced German pickles and chopped onions on each beef slice.

Roll up the beef slices, tucking in the sides as best you can and securing the beef rolls with toothpicks or cooking twine.

Heat the butter and oil in a heavy Dutch oven or pot and generously brown the rouladen on all sides. Set the rouladen aside on a plate.

Add the onions to the pot and a little more butter or oil if needed. Cook the onions until softened and translucent, about 5 minutes. Add the garlic and cook for another minute. Add the leek, carrots and celery and cook for another 5 minutes.

Pour in the red wine, bring to a rapid boil for one minute, reduce the heat to medium and simmer for 2-3 more minutes. Add the beef broth, tomato paste, bay leaf, sugar, salt and pepper.

Nestle the beef rouladen in the pot. You can cook the rouladen, covered, on the stovetop on low for about 90 minutes or until fork tender, or transfer the pot to the oven preheated to 325 F and cook it there for about 90 minutes or until fork tender.

When the beef is fork tender, remove the rouladen from the pot and set aside. Pour the liquid and vegetables through a strainer and reserve the liquid.

Return the strained liquid back to the pot and bring to a simmer. Thicken the gravy either with either a cornstarch slurry or flour slurry. For a creamy gravy you can also add a few tablespoons of heavy cream at this point.

Add the chilled butter, whisking constantly, until the butter is melted and incorporated. Add salt, pepper and mustard to taste.

Carefully remove the toothpicks or cooking twine from the rouladen and return them to the gravy and heat through.

Serve the rouladen and gravy with Homemade Rotkohl and either Homemade Spätzle, Homemade SemmelKnödel or boiled potatoes.