Lay out the Beef Rouladen and pat dry. Lightly season each slice with Salt and freshly ground Black Pepper, then spread a thin layer of German Mustard over the meat.
Place a strip of Streaky Bacon, a few lengthwise slices of Dill Pickles and a few thin rings of Onion near one edge of each slice. Roll up tightly and secure with kitchen twine or toothpicks. Dredge each roulade lightly in Flour and shake off excess.
Heat a heavy-bottomed pot or Dutch oven with about 1–2 tablespoons of Lard over medium-high heat. Brown the rouladen on all sides until a deep, caramelized color develops, then remove and set aside.
Reduce heat to medium and add the chopped Onion (if you have more), plus diced Carrot, Celery and Leek to the pot. Sauté until softened and starting to color, about 6–8 minutes.
Stir in the Tomato Paste and cook 1–2 minutes to deepen the flavor, then pour in the Red Wine to deglaze the pan, scraping up all browned bits from the bottom.
Return the rouladen to the pot and add enough Beef Stock to come about halfway up the meat. Tuck in Bay Leaves, a few crushed Juniper Berries and several Black Peppercorns. Bring to a gentle simmer, cover, and braise on low heat for 1½–2 hours, or until the beef is very tender.
When the rouladen are done, transfer them to a plate and keep warm. Strain the braising liquid through a sieve into a clean saucepan, pressing the vegetables to extract their juices; discard solids and skim any excess fat.
Bring the strained sauce to a simmer and finish with a knob of Butter for shine. If the sauce needs thickening, whisk together a small amount of Cornstarch with cold Water to make a slurry, then whisk it into the simmering sauce a little at a time until it reaches the desired consistency. Adjust seasoning with Salt and Black Pepper.
Slice the rouladen, arrange on plates and spoon the warm sauce over the top. Serve immediately with traditional sides like boiled potatoes, mashed potatoes or braised red cabbage.
