Season beef medallions on all sides with salt and pepper; set aside.
In a large sauté pan, heat the olive oil over medium-high heat until hot.
Add beef medallions; sear for 3 minutes per side for medium-rare, 4 minutes per side for medium and 5 minutes per side for well done.
Transfer medallions to a plate and tent with foil to keep warm.
In the same pan, add the mushrooms; cook and stir for 3 to 4 minutes.
Next, add shallot and garlic; blend them into the mushrooms and cook for 1 to 2 minutes.
Add the vinegar; cook until it has completely evaporated. Add broth; bring to a boil and cook for 2 to 3 minutes.
Whisk in sour cream; cook for just 1 minute then immediately remove the pan from the heat.
Stir in chopped parsley. Serve medallions topped with hot mushroom sauce.