Slice the beef across the grain into pieces about 1 - 1.5 inches long
Melt the butter in a large skillet over med-high heat. Add the sliced beef, and cook, stirring occasionally, until the beef is browned.
Stir 1-1/4 cups worth of the beef broth, tomato paste, and salt into the skillet. Bring to a boil, and then reduce heat to low. Cover and simmer for about 10 min (until the beef is tender).
Stir in the mushrooms, onions, garlic, & oregano. Cover again and continue to simmer until the onions are tender (about 5 minutes).
In a small bowl, stir together the remaining 1/2 cup of beef broth and arrowroot powder until combined.
Stir the arrowroot mixture into the skillet. Bring to a boil while stirring. Boil for 1 minute, until thickened.
Stir in the sour cream and heat through. Serve immediately.