Heat the butter in a large pan on medium-high heat. Once butter has melted, add the onion, garlic, mushrooms, salt, and the pepper. Saute for 5-7 mins, stirring regularly.
Add the beef and flour to the pan. Whisk until well mixed into the veggies. Keep stirring constantly, for at least 5 mins.
Meanwhile, in a separate bowl or pyrex, whisk together the beef broth, Worcestershire sauce, and smoked paprika. Slowly add the broth mixture to the pan and simmer beef mixture together for 5 minutes.
As a final step, stir in the sour cream and stir/whisk until evenly distributed, until it's as thick as you like. Scoop beef mixture on top of pasta for rice. Optional: Sprinkle with fresh, chopped chives.
To keep sirloin tender, be sure to cut along the grain.
Buy pre-sliced ingredients to save time on prep work. Simply ask your butcher to slice your beef and/or buy pre-sliced mushrooms. That way, the only prep you'll need to do is chop the onion and garlic.
To keep mushrooms firm (and not soggy!) be careful not to overcook them in the butter.
Sauce will thicken as it cooks. Allow sauce to simmer until you reach your desired thickness.