Heat 2 tablespoons of olive oil in a large, deep pan over medium-high heat. Sear 450g of top sirloin steak, thinly sliced into strips, in two batches for about 1 minute per side, until browned. Set aside, covered.
Melt 2 tablespoons of butter in the same pan. Cook ½ chopped onion and 225g of sliced mushrooms for 6-8 minutes, until their liquid has cooked off.
Stir in 1 minced garlic clove, then 1 tablespoon of all-purpose flour. Add 1 cup of beef broth, scraping up the browned bits, then ¾ cup of heavy cream. Simmer until slightly thickened.
Mix a little of the pan sauce into ¼ cup of sour cream, then stir it back in with 1 tablespoon of Worcestershire sauce, ½ teaspoon of Dijon mustard, ½ teaspoon of salt, and ¼ teaspoon of black pepper. Return the beef and heat through.
