In a large skillet or frying pan, heat the olive oil over medium-high heat.
Add the sliced beef to the pan and cook until browned on all sides, about 5-7 minutes.
Remove the beef from the pan and set aside.
In the same pan, add the chopped onion and minced garlic. Cook until softened, about 3-4 minutes.
Add the sliced mushrooms to the pan and cook until tender, about 5 minutes.
Stir in the flour and cook for 1-2 minutes, until the flour is lightly browned.
Gradually add the beef stock, stirring constantly, until the mixture thickens, about 5-7 minutes.
Stir in the paprika, Dijon mustard, tomato paste, salt, and pepper, until well combined.
Add the beef back to the pan and stir to coat with the sauce.
Gently stir in the sour cream and cook for 5 minutes.
Serve the beef stroganoff over egg noodles or rice, and garnish with chopped parsley, if desired.
