Heat 1 tablespoon of olive oil in a large skillet over medium-high heat.
Season the sliced beef with salt and pepper.
Brown the beef slices on all sides for 2-3 minutes per side.
Remove the beef from the skillet and set it aside.
In the same skillet, add 1 tablespoon of butter.
Sauté the onions until they are soft, then add garlic and mushrooms.
Cook for about 4-5 minutes until the vegetables are tender.
Pour in the beef stock and bring to a simmer.
Stir in the Dijon mustard.
Let it simmer until the sauce reduces slightly, about 5 minutes.
Reduce the heat to low, then stir in the sour cream.
Mix until the sauce is creamy and smooth.
Season with more salt and pepper to taste.
Add the beef back into the skillet.
Cook for an additional 2-3 minutes to heat through.
Serve over egg noodles.