Pat the beef roulades dry with paper towels. Season both sides with salt, pepper, and mustard.
Keep the bacon fat cold and cut into very thin strips. Cut the pickled cucumbers and one of the large onions into strips.
Lay each roulade flat on a work surface. Place bacon strips, cucumber strips, and onion strips on each roulade. Roll up tightly and tie with kitchen string at several points to secure.
Sprinkle the tied roulades with sweet paprika powder.
Heat margarine in a large pot over medium-high heat. Fry the roulades for 8-10 minutes, turning occasionally, until browned on all sides. Remove to a plate.
In the same pot, add the remaining large onion (roughly chopped) and the carrot (roughly chopped). Fry over medium heat for about 5 minutes.
Add a dollop of tomato paste from the tube and stir well. Cook for 2-3 minutes to deepen the flavor. Pour in enough water to come about halfway up the sides of the pot.
Stir in the bay leaf, 1 pinch of dried marjoram (you should barely taste it), salt to taste, and a small amount of dried celery leaves (not too much). Return the roulades to the pot.
Bring to a boil, then reduce heat to low. Simmer gently for 1.5 to 2 hours, until the beef is very tender when pierced with a fork.
Just before serving, season with ground pepper and add lemon juice to taste.
Optional—if you want a thicker sauce, stir in a sauce thickener according to package directions (not necessary as the onion naturally thickens the cooking liquid).