Pat the beef roulades dry. Pound them wider and thinner if necessary. Brush mustard on one side of each roulade and season both sides with salt and pepper.
Cut the bacon and pickled cucumbers into thin slices. Dice one of the 2 large onions. For each beef roulade, arrange bacon slices, cucumber slices, and diced onion in the center, roll up tightly, and secure with a roulade stick.
Sprinkle the rolled roulades with paprika powder. Heat fat in a large pot or Dutch oven over medium-high heat and brown the roulades on all sides, about 5-7 minutes until deeply browned.
Remove the roulades and set aside. In the same pot over medium heat, add the diced second large onion and the finely chopped small carrot. Fry for 3-4 minutes until beginning to soften.
Stir in a dollop of tomato paste and 1 teaspoon mustard. Cook for 1 minute until the paste darkens slightly.
Pour in the shot of red wine and use a wooden spoon to scrape the bottom of the pot, releasing any browned bits. Simmer for 2 minutes.
Add 1 liter of broth, 1 bay leaf, 1 pinch of dried marjoram, and 1 pinch of sugar. Stir to combine and bring to a simmer.
Return the browned roulades to the pot, nestling them in the liquid. Cover, reduce heat to medium-low, and braise for 2 hours.
Remove the lid, increase heat to medium, and simmer uncovered for 15-20 minutes to reduce and concentrate the sauce.
Remove from heat. Season with pepper to taste and add lemon juice to taste. Stir in a piece of cold butter until melted and combined. Whisk in dark sauce thickener if desired to thicken the sauce.
Serve the beef roulades with sauce spooned over, accompanied by mashed potatoes and red cabbage.