Heat 2 tablespoons of olive oil over medium-high heat in a large pot. Season 1 pound of flank steak with salt and pepper, then add it to the pot, browning each side for about 4-5 minutes until a nice crust forms. Remove the steak from the pot and set aside.

Lower the heat to medium and add 2 tablespoons of unsalted butter to the pot. Once melted, add 1 chopped onion and cook for about 3 minutes until translucent. Stir in 2 minced garlic cloves and cook for an additional 30 seconds until fragrant.

Sprinkle in 1/4 cup of all-purpose flour, stirring well to combine with the butter and aromatics. Cook this roux for about 1 minute, then gradually whisk in 4 cups of beef broth.

Add 2 cups of diced Yukon Gold potatoes and 2 tablespoons of Worcestershire sauce, bringing the mixture to a boil. Once boiling, reduce heat to a simmer and cover, cooking for approximately 15 minutes until the potatoes are tender.

Return the reserved steak to the pot, along with any juices. Stir in 1 cup of heavy cream and simmer for an additional 5 minutes.

Serve the soup in bowls, optionally topped with chopped chives or shredded cheddar cheese.