In a mixing bowl, combine the sliced beef with the flour, salt, and pepper. Toss the beef until fully coated with the mixture.
Heat the olive oil in a large skillet over medium-high heat. Add the onion, garlic, and mushrooms, and cook until they soften and the onions become translucent.
Remove the vegetables from the skillet and set them aside.
In the same skillet, melt the butter over medium-high heat. Add the beef and cook for 2-3 minutes on each side until browned. Remove the beef from the skillet and set it aside.
Add the cognac to the skillet and cook until it’s reduced by half.
Add the beef broth, Dijon mustard, and tomato paste to the skillet, stirring until fully combined.
Add the beef and vegetables back to the skillet and stir to coat them in the sauce.
Reduce the heat to low and let the beef mixture simmer for 10-15 minutes until the sauce has thickened.
Remove the skillet from the heat and stir in the sour cream.
Serve the beef stroganoff over egg noodles or rice, and garnish with chopped parsley, if desired.
