Cut sirloin into ¾ inch pieces (bite sized). Stir the flour and salt together in a medium sized bowl. Add the beef and stir to coat.
Melt the butter in a large skillet over medium heat. Add the beef and let cook, without stirring, until browned on the bottom (about 2–3 minutes).
Flip and cook on the other side. Remove the beef from the skillet and set aside.
Add the chopped onion and mushrooms to the pan (no need to wipe the skillet first). Cook for 4–5 minutes, or until the onions are soft and the mushrooms are nearly cooked through.
Add the beef back to the pan, along with the garlic powder, cream of mushroom soup, water, and tomato paste. Stir to combine.
Cook until the meat is cooked through yet still tender, about 5–6 minutes. Take off the heat.
Stir in the sour cream (doing this last prevents it from curdling).
Serve with egg noodles, rice, or mashed potatoes.
Video
Notes
I used top sirloin for this recipe because it's economical and stays tender (provided you don't overcook it!). You can also use boneless ribeye, beef tenderloin, or even lean ground beef. Don't love mushrooms? You can leave them out. Leftovers will keep for 4–5 days in the refrigerator. Reheat the stroganoff in a skillet or in the microwave. This dish also freezes well.