Heat the olive oil and butter in a large skillet over medium heat.
Add the onion and garlic, sauté until soft and fragrant.
Add the mushrooms and cook until golden brown.
Add the thinly sliced ​​beef and cook until golden brown. Remove the meat and set aside.
Sprinkle the flour over the vegetables and mix well to make a roux. Slowly add the beef stock, stirring to avoid lumps.
Return the meat to the skillet, add the crème fraîche and mustard. Simmer for 5 minutes until the sauce thickens.
Season with salt and pepper. Garnish with fresh parsley and serve immediately.
Enjoy!