Step 1: Prepare the Cheese Mixture - In a small bowl, combine the grated Parmesan cheese, mozzarella cheese, cream cheese, and mustard. Mix until smooth and well blended. Set aside.
Step 2: Make the Crumb Topping - In another bowl, combine the bread crumbs, garlic powder, fresh parsley, and one tablespoon of butter. Work the mixture together until it resembles a sandy, crumb-like texture. Set aside.
Step 3: Season the Steak - Pat the sirloin steak dry with a paper towel. Season both sides generously with salt and black pepper to enhance the flavor.
Step 4: Sear the Steak - Heat a skillet over medium-high heat. Add the steak and cook for about four minutes until nicely browned. Flip the steak, add one tablespoon of butter, and cook for another four minutes while basting with the melted butter.
Step 5: Assemble the Crust - Preheat the oven to 180°C or 350°F. Transfer the steak to a baking tray. Spread the prepared cheese mixture evenly over the top, then layer the crumb topping over it.
Step 6: Bake to Perfection - Bake for about ten minutes, or until the topping is golden and crispy.
Step 7: Rest and Serve - Allow the steak to rest for five minutes before slicing. This helps the juices redistribute for maximum tenderness and flavor.