In a large skillet, heat olive oil and butter over medium-high heat.
Brown the thin-sliced beef on both sides, then remove it from the pan and set it aside.
Add sliced onion and mushrooms to the pan drippings.
Cook them for about 5 minutes until the onion becomes soft and mushrooms turn golden.
Sprinkle the vegetables with flour and stir well to combine.
This helps thicken the sauce later on.
Return the beef to the skillet and add the mushroom soup and beef broth.
Mix everything, cover the pan, reduce the heat, and let it simmer for about 30 minutes.
In the last few minutes of cooking, stir in the sour cream.
Season with salt and pepper to your liking, and serve over cooked noodles.