8slices top round beef, about 4x6 inches in size and 1/4 inch thick (*see note)(ask your butcher), gently pound the slices with a meat mallet until they're a little thinner than 1/4 inch (be careful not to pound holes into them)
cornstarch or flour dissolved in a little water for thickening(depending on how much gravy liquid there is you'll need about 1-2 tablespoons of cornstarch dissolved in 2-3 tablespoons of water or 2-3 tablespoons flour dissolved in 1/4 - 1/3 cup water)
Lay the beef slices out on a work surface. Spread each beef slices 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 (make sure it's oven-safe if baking in the oven) and generously brown the rouladen on all sides. Browning them well will ensure a rich and flavorful gravy. Set the rouladen aside on a plate.
*Do not remove the browned bits in the bottom of the pan (important for a flavorful gravy): 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. Oven or Stovetop: You can cook the rouladen, covered, on the stovetop on low for about 90 minutes or until fork tender, but for the most even cooking we recommend transferring the pot (make sure it's oven-safe) to the oven preheated to 325 F and cook it there for about 90 minutes or until fork tender.
To Make the Rouladen Gravy: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. (You can eat the veggies on the side or puree them in the blender and then return them to the gravy.) Return the strained liquid back to the pot and bring to a simmer. Thicken the gravy either with either a cornstarch slurry (for a clear/translucent gravy) or flour slurry (for an opaque gravy). For a creamy gravy you can also add a few tablespoons of heavy cream at this point. Simmer, whisking constantly, until the gravy is thickened.Add the chilled butter, whisking constantly, until the butter is melted and incorporated. Add salt, pepper and mustard to taste. Note: If you prefer a creamy gravy you can stir in some heavy cream.Carefully remove the toothpicks or cooking twine from the rouladen and return them to the gravy and heat through.