Chop the onions.

Cut the beef into thin strips.

Cube the potatoes.

Sauté the chopped onions in olive oil in a large pot for 10 minutes over medium heat, until softened.

Add the beef strips to the pot with the onions and sauté for another 5 minutes, browning the meat.

Pour in the beef stock and bring the mixture to a simmer.

Reduce heat and let it simmer for 1 hour, allowing the flavors to meld.

Add the cubed potatoes, salt, pepper, and bay leaves to the soup.

Continue to simmer for an additional 20 minutes, or until the potatoes are tender.

Remove the bay leaves from the soup.

In a separate small bowl, remove 1/2 cup of broth from the soup.

Slowly whisk in the flour into the removed broth until smooth, creating a slurry.

Gradually pour the flour mixture (slurry) back into the main pot of soup, stirring constantly to avoid lumps.

Simmer the soup for 3 minutes to allow the flour to cook and thicken the soup slightly.

Stir in the vinegar, cream (if using), and chopped parsley.

Stir well to combine all ingredients.

Remove the pot from the heat.

Serve hot in bowls.