Season flanken and beef with salt and pepper to taste. In a pot, heat oil over high heat and sear meat until golden brown on all sides, one to two minutes per side. Remove from pot and set aside.

Reduce heat to medium. Add onions and sauté until translucent, about three to four minutes.

Cut five carrots and two zucchini into chunks and add them to the pot. Add celery, tomatoes, and potatoes and sauté five minutes.

To the net cooking bag, add remaining two whole carrots, remaining one whole zucchini, and parsley roots. Place bag in the pot. Return meat to the pot.

Add stock, two tablespoons salt, and water to cover. Bring to a boil, then reduce heat to low and simmer four hours.

Remove cooking bag. Using a blender (or a bowl with high sides and an immersion blender), purée the vegetables from the bag. Add purée to the pot and stir to combine. Serve with a piece of meat in each bowl.