Rub both sides of the sirloin steaks with steak seasoning.
Heat olive oil in a large skillet over medium-high heat. Add the steaks and cook for 3–4 minutes per side, or until they reach your desired doneness. Remove from the skillet and let rest on a plate, loosely covered.
Pour the beef broth into the skillet and scrape up any browned bits from the bottom.
Add the minced onion and sliced mushrooms to the broth. Cook for 5–6 minutes, stirring occasionally, until the liquid reduces and the vegetables are softened and lightly golden.
Reduce heat to low. Stir in the sour cream until smooth and heated through.
Spoon the mushroom sauce over the steaks and serve immediately.