Lay each beef slice flat.

Spread mustard evenly over each slice.

Season with salt and pepper.

Top each slice with a bacon slice.

Place chopped onions and a pickle spear at one end of each slice.

Roll up each slice tightly, tucking in the sides.

Tie butcher string around each rouladen to secure the filling.

Heat olive oil in a large pan over medium-high heat.

Brown the rouladen on all sides.

Remove any excess oil from the pan.

Add beef stock to the pan and bring to a simmer.

Cover and simmer on medium-low heat for 90 minutes, or until fork-tender.

Remove rouladen from the pan and let rest for 10 minutes, covered loosely with foil.

Whisk together cornstarch and cold water until smooth.

Bring the pan juices to a simmer.

Slowly add the cornstarch mixture while stirring continuously.

Bring to a boil, then reduce heat and simmer until the starchy flavor is gone, about 10 minutes.

Add red wine (if using) near the end for extra flavor.

Serve the rouladen with the pan gravy.