Preheat your oven to 375°F (190°C). Season the cut pieces of sirloin steak generously with salt and black pepper. In a skillet over medium-high heat, add olive oil and sear the steak pieces for about 2-3 minutes on each side until browned. Remove from skillet and set aside.
In the same skillet, reduce the heat to medium and add butter until melted. Add minced garlic and cook for about 30 seconds. Sprinkle in all-purpose flour to create a roux, cooking for around 1 minute before gradually whisking in milk and beef broth until smooth. Stir in shredded cheddar cheese, Dijon mustard, and paprika until creamy.
Bring a pot of salted water to a boil and cook cheese tortellini according to package directions but slightly undercooking them. Drain well and toss with a little olive oil.
Grease a 9×13 inch baking dish with cooking spray or butter. Combine cooked tortellini, seared steak pieces, and creamy cheese sauce, mixing gently. Top with shredded mozzarella cheese.
Place the assembled casserole into the preheated oven uncovered and bake for about 20-25 minutes until cheese is bubbling and golden brown. Sprinkle chopped parsley over it before serving warm.