Heat olive oil and butter in a large skillet over medium-high heat.
Add the beef strips and brown them on both sides.
Remove from the skillet and set aside.
Add the sliced onions and mushrooms to the skillet with the pan drippings.
Sauté until the onions are translucent and the mushrooms are tender.
Sprinkle 2 tablespoons of flour over the onion and mushroom mixture.
Stir constantly to cook the flour for about one minute to remove any raw flavor.
Stir in the mushroom soup and beef broth, ensuring it mixes evenly with the flour and vegetables.
Bring the mixture to a gentle boil, allowing it to thicken slightly.
Reduce the heat to low.
Return the browned beef to the skillet and mix in the sour cream.
Simmer gently for about 10-15 minutes, until the beef is fully cooked and tender.
Adjust seasoning with salt and pepper.