Cook bacon in a large enameled cast iron pot such as a dutch oven. Cook bacon until fat has rendered but bacon is still soft, not crisp. Remove bacon from pan and drain on paper towels.
In the same pan cook the diced onion in the bacon fat until soft and translucent. Remove onions from the pan and set aside.
Remove pan from heat, reserve about 1 tablespoon of the remaining bacon fat in the pan, discarding any excess.
On a large cutting board lay out the slices of beef.
Season generously with salt, pepper and paprika.
Spread a thin layer of mustard onto each slice of beef.
Add an even layer of the cooked bacon and onions to each slice of beef.
Place a pickle spear on each piece of beef.
Roll up slices of beef, tucking the ends in and securing the rolls with either skewers, wooden cocktail picks, or string.
Dredge each roll in flour to coat; shake off excess flour.
Reheat the reserved tablespoon of bacon fat left in the dutch oven.
When bacon fat is hot add the flour coated rolls of beef to the pot.
Use tongs to turn the rolls and brown them on all sides.
When the rolls are nicely browned remove the rolls from the pan, and set aside.
Add beef broth, beef consumme and sliced mushrooms to the pot and bring to a boil. Scrape up browned bits at the bottom of the pan.
Once mixture comes to a simmer, return beef rolls to the pot.
Stir gently to coat beef rolls with gravy and ensure they are submerged in the gravy. Add water if needed.
Cover pot with lid and reduce heat to low.
Cook on low for 1½ to 2 hours, stirring occasionally, until beef is soft and tender and cooked through.
In a small bowl combine ½ cup water and cornstarch.
Add cornstarch slurry to the pot and bring up heat to maintain a simmer. Cook for 5-10 minutes or until gravy thickens to desired consistency.
Remove picks and/or string and serve Rouladen over a bed of mashed potatoes and garnish with freshly chopped parsley.