On a cutting board lined with plastic wrap or parchment paper, lay out beef slices in a single layer. Cover with a sheet of plastic wrap, and gently pound slices using a meat tenderizer.
Spread a thin layer of brown mustard over each slice, about 1 1/2 teaspoons each. Lay one bacon slice over each slice of beef, trimming if necessary. Top each with a small amount of the sliced onions and sprinkle with salt and freshly ground back pepper. Top with a pickle spear, if using.
Roll each rouladen jelly-roll style, and secure with twine.
In a large Dutch oven or heavy-bottomed saucepan, heat butter over medium-high heat. Brown each roll, about 2 to 3 minutes per side. Remove rouladen, set on a plate, and cover to keep warm. Add remaining onions, garlic, leeks, carrots, celery to pot and saute until the onions start to become translucent, about 5 minutes.
Add red wine to Dutch oven, deglaze pan making sure to scrape up any brown bits. Stir in beef broth, tomato paste, bay leaf, sugar, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Return rouladen to pan, cover, reduce heat and simmer until fork-tender, about 60 to 90 minutes.
Remove rouladen from pot, set aside. Increase heat to medium-high and bring broth to a boil. In a small bowl, combine cornstarch with water to make a slurry. Add cornstarch slurry to the boiling broth while whisking until it reaches the desired thickness. Remove bay leaf and season to taste with salt and pepper.
Remove twine from rouladen. Garnish with chopped fresh parsley. Serve over mashed potatoes topped with gravy, if desired.