Season steak with ¼ teaspoon each of salt and pepper.
Heat oil in a large skillet over medium-high heat. Cook steak in small batches until brown (1-2 minutes per side). It does not have to be cooked through, it will finish cooking in the sauce. Transfer steak to a bowl and cover to keep warm.
Reduce heat to medium. In the skillet whisk together the cream of mushroom soup, beef broth, mustard, garlic powder, and ¼ teaspoon pepper and cook until hot and bubbling.
Reduce heat to low and stir in steak and sour cream. Cook for 1-2 minutes more until steak is heated through and sour cream is fully combined. Taste and season with additional salt and pepper as needed.
Notes
Cut steak across the grain to get the most tender pieces. Leftovers will keep in an airtight container in the refrigerator for 4 days and in the freezer for 4 months.