For the rouladen: Lay out the beef slices on a flat surface and gently pound them with a meat mallet until they’re a little thinner than ¼". Take care to not pound holes in the beef. Season beef with salt and black pepper, to taste.

Spread a tablespoon of yellow mustard evenly onto each beef slice, and then add two slices of bacon. Place sliced pickle horizontally on one end of each beef slice, then sprinkle chopped yellow onion over the top of the entire beef slice. Repeat this layering of ingredients with all beef pieces.

Starting from the end with the pickle, roll up each beef slice tightly. Secure each roll with a toothpick. 

In a Dutch oven or large skillet (make sure you have a lid that fits) over medium-high heat, melt the butter with the olive oil.

Add the rouladen to the Dutch oven and brown them on all sides until they are nicely seared. Once browned, remove them from the skillet and set aside. 

For the gravy: To the same Dutch oven, add the chopped onion, carrot, and celery. Sauté until the vegetables are tender. Then stir in the garlic and sauté for 1 more minute.

Stir in the soy sauce, beef broth, and tomato paste. Bring the mixture to a simmer.

Return the browned rouladen to the Dutch oven, making sure they are submerged in the liquid. If needed, add more beef broth to cover the rouladen.

Reduce the heat to low, cover the Dutch oven, and let it simmer gently for 1½ to 2 hours, or until the beef is absolutely tender. Then remove the rouladen from the Dutch oven, and remove the toothpicks. 

Carefully pour the hot cooking broth and vegetables through a strainer set over a bowl, and reserve the cooking broth. Return the strained cooking broth back to the Dutch oven and bring to a simmer. 

In a small cup or bowl, mix the cornstarch and water to make a slurry. Slowly pour the cornstarch slurry into the simmering cooking broth, whisking continuously. Let simmer for a few minutes, until the gravy thickens to your desired consistency. If you'd like it thicker, make another batch of slurry and repeat. Taste test the gravy, then season with salt and black pepper if needed.

Remove the Dutch oven from the heat. Then sir in the chilled butter until it melts and gives the gravy a glossy finish.

To serve: Place the rouladen in a low wide serving bowl or platter. Spoon the gravy over the rouladen. If desired, garnish with chopped fresh parsley.