Heat a large skillet or Dutch oven over medium-high heat. Season flank steak with salt and black pepper. Sear each side for about 3 minutes until golden brown. Remove steak and let rest before slicing thinly against the grain.

Reduce heat to medium in the same skillet, melt 2 tablespoons of butter. Add diced onion and sliced mushrooms, cooking for 7-9 minutes until onion is translucent and mushrooms are golden. Stir in minced garlic and cook for an additional minute.

Pour 4 cups of beef broth into the skillet, bring to a simmer. Stir in 1 cup of heavy cream and let simmer for 10-12 minutes until thickened.

In a separate pan, melt 2 tablespoons of butter over medium heat. Sauté thinly sliced cabbage for 6-8 minutes until soft. Season with salt and black pepper.

Return sliced flank steak to the soup, stirring to combine. Allow to warm through for 2-3 minutes.

Ladle soup into bowls, topping with sour cream, sautéed cabbage noodles, and parsley before serving.