Boil the Pasta: Bring a large pot of salted water to a boil. Add the rigatoni and cook according to package instructions until al dente. Drain well and set aside.
Prep the Meat: Pat the sirloin steak cubes completely dry with a paper towel. Season generously with salt and pepper.
Sear the Steak: Heat a large skillet over medium-high heat. Once very hot, add the steak bites in a single layer (work in batches if needed to avoid overcrowding). Sear for 2-3 minutes per side until a dark, caramelized crust forms.
Add Garlic Butter: Lower the heat to medium. Add the butter and minced garlic to the skillet. Toss the steak in the melting garlic butter for about 1 minute until fragrant. Remove the steak bites from the skillet and set aside on a plate.
Start the Sauce: In the same skillet over medium-low heat, pour in the heavy cream. Use a wooden spoon to scrape up any flavorful browned bits left from the steak. Let the cream simmer for 2-3 minutes.
Melt the Cheese: Stir in the mozzarella and grated Parmesan cheese. Stir continuously until the cheeses are completely melted and the sauce is smooth and thick. Taste and adjust salt and pepper if needed.
Combine and Serve: Add the cooked rigatoni directly into the cream sauce, tossing well so the pasta is fully coated. Serve immediately, topped with the garlic butter steak bites.