Season steak slices with salt and pepper.
In a large skillet, heat 2 tablespoons of oil over medium-high heat.
Add steak and sear until browned. Remove and set aside.
Use the same skillet and add the remaining 2 tablespoons of oil.
Sauté onions until they turn translucent.
Add mushrooms and garlic, and continue sautéing until mushrooms are browned.
Sprinkle flour over mushrooms and stir well.
Gradually add beef broth, stirring constantly. Scrape the skillet bottom to loosen any bits.
Reduce heat and simmer until the sauce thickens.
Mix in sour cream (or Greek yogurt), Worcestershire sauce, and mustard.
Return the steak to the skillet. Stir to combine.
Let it simmer for 2-3 minutes or until the steak reaches your preferred doneness.
Serve the stroganoff over fluffy rice.
