1. Start by seasoning the thinly sliced sirloin steak on both sides with salt and ground black pepper to taste.

2. In a large skillet, heat 2 tablespoons of vegetable oil over medium-high heat.

3. Add the seasoned steak slices in a single layer, searing them for about 2-3 minutes per side until well browned.

4. Once browned, remove the steak from the skillet and set it aside for later.

5. Using the same skillet, pour in the remaining 2 tablespoons of vegetable oil.

6. Add the diced yellow onion and sauté over medium heat until translucent, about 2-3 minutes.

7. Next, add the sliced baby bella mushrooms and minced garlic.

8. Continue cooking for about 5 minutes until the mushrooms soften and begin to brown.

9. Sprinkle the all-purpose flour evenly over the mushroom mixture.

10. Stir thoroughly so the flour is well incorporated and coats the vegetables.

11. Gradually add the beef stock while stirring constantly, making sure to scrape up any browned bits from the bottom of the skillet.

12. Bring the mixture to a simmer and cook for several minutes until the sauce thickens.

13. Lower the heat and mix in the sour cream (or plain Greek yogurt), Worcestershire sauce, and Dijon mustard until the sauce is smooth and creamy.

14. Return the seared steak from Step 1, along with any accumulated juices, to the skillet.

15. Stir to combine all the ingredients.

16. Allow the mixture to simmer for 2-3 minutes, or until the steak is heated through and reaches your preferred level of doneness.

17. Spoon the finished stroganoff generously over freshly cooked rice.