Season beef medallions generously with salt and pepper on all sides.
Heat olive oil in a large heavy-bottomed skillet over medium-high heat until shimmering.
Sear medallions for 3 to 4 minutes per side until reaching an internal temperature of 130 degrees Fahrenheit for medium-rare.
Remove beef to a plate and tent loosely with aluminum foil to rest.
Reduce heat to medium, add butter to the skillet, and sauté minced shallots for 2 minutes until translucent.
Deglaze the pan with white wine, scraping the bottom to release the fond, and simmer until liquid is reduced by half.
Whisk in beef stock, heavy cream, Dijon mustard, and whole grain mustard.
Simmer the sauce for 3 to 5 minutes until it reaches a nappe consistency.
Return medallions and any accumulated juices to the pan to coat with sauce.
Garnish with fresh parsley and serve immediately.