Steps

• Begin by slicing the filet mignon into thin strips, ensuring that you cut against the grain to keep the meat tender.
• Heat the olive oil and butter in a large sauté pan over medium heat. Add the sliced onions and mushrooms, stirring frequently. Cook until the mushrooms release their moisture and the onions are golden brown, about 10 minutes.
• Season the filet strips with salt and pepper. Increase the heat to high and add the meat to the pan, cooking it quickly to seal in the juices. This should only take about 2-3 minutes. Remove the meat and set aside.
• Reduce the heat to medium, then sprinkle flour over the onion and mushroom mixture. Stir briskly to prevent clumping. Slowly add beef broth while continuously stirring, then whisk in the dijon mustard. Bring the mixture to a gentle simmer until it thickens, about 5 minutes.
• Return the cooked filet pieces to the pan and coat with the sauce. Turn off the heat and gently fold in the sour cream, ensuring the sauce becomes silky and envelops the filet. Stir to combine thoroughly.