Brown the beef: In a large skillet, heat the olive oil or butter over medium-high heat. Add the sliced beef, season with salt and pepper, and cook until browned on all sides, about 5-7 minutes. Remove the beef and set aside.

Cook the onions and mushrooms: In the same skillet, add the chopped onions and garlic. Cook for 3-4 minutes until softened. Add the sliced mushrooms and cook for another 5 minutes until tender and browned.

Deglaze with beef broth: Stir in the beef broth, scraping up any browned bits from the bottom of the skillet. Let the mixture simmer for 3-5 minutes to reduce slightly.

Add cream and mustard: Lower the heat to medium and stir in the heavy cream, sour cream, and Dijon mustard (if using). Let the sauce simmer and thicken for 5 minutes.

Return the beef to the skillet: Add the browned beef back into the skillet, stirring to coat the beef in the sauce. Simmer for another 5 minutes to warm through.

Serve: Garnish with fresh parsley and serve over zucchini noodles or cauliflower mash for a keto-friendly meal.