1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish and set aside.
2. Cook the tortellini according to package instructions until just al dente, about 2-3 minutes less than recommended. Drain and set aside. I like to undercook pasta slightly here because it will continue cooking in the oven, preventing a mushy texture.
3. Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Season the sliced steak generously with salt, pepper, and smoked paprika.
4. Sear the steak in batches for about 2 minutes per side until browned but still tender inside. Remove from skillet and set aside.
5. In the same skillet, add the remaining tablespoon of olive oil and reduce heat to medium. Sauté the chopped onion until softened and translucent, about 5 minutes.
6. Add the minced garlic and cook for another 30 seconds until fragrant.
7. Stir in the butter until melted, then sprinkle the flour over the onion and garlic mixture. Cook, stirring constantly, for 1-2 minutes to create a roux. This step is key—I learned early on that skipping it can lead to a runny sauce, which is no good for a casserole.
8. Gradually whisk in the milk, making sure to break up any lumps. Continue cooking and stirring until the sauce thickens and coats the back of a spoon, about 5 minutes.
9. Remove the skillet from heat and stir in half of the mozzarella, cheddar, and Parmesan cheeses. Season with Italian seasoning, additional salt, and pepper to taste.
10. Combine the cooked tortellini, seared steak slices, and cheese sauce in a large bowl. Mix gently to coat everything evenly.
11. Pour the mixture into the prepared baking dish and top with the remaining shredded cheeses.
12. Bake uncovered for 20-25 minutes, or until the cheese is melted, golden, and bubbly. For an extra golden crust, you can broil for 2-3 minutes at the end—just watch closely to avoid burning.
13. Allow the casserole to rest for 5 minutes before serving. Garnish with fresh parsley or basil if desired for a pop of color and freshness.

One of my favorite moments cooking this is when that golden crust forms on top—the sign that all the flavors have melded together perfectly. It always reminds me of the family dinners I grew up with, where the simple act of sharing food created lasting memories.