Bring a large pot of salted water to a rolling boil over high heat. Add rigatoni pasta and cook according to package instructions until al dente, usually about 10-12 minutes. Once cooked, drain the pasta in a colander and set it aside.

In a large skillet, melt 2 tablespoons of butter over medium-high heat. Season sliced steak with salt and pepper, then add it to the hot skillet. Cook the steak for 3-4 minutes, or until browned and cooked to your desired doneness. Remove the steak from the skillet and set it aside.

Lower the heat to medium and add the remaining 2 tablespoons of butter to the skillet. Once melted, toss in minced garlic and red pepper flakes. Sauté for about 1 minute until fragrant and lightly golden.

Pour in beef broth and bring the mixture to a gentle simmer, cooking for 2-3 minutes. Stir in heavy cream and continue to simmer gently for another 2-3 minutes, allowing the sauce to thicken.

Return the cooked steak to the skillet, then add the drained rigatoni and grated Parmesan cheese. Toss everything together until well combined and the pasta is coated in the sauce. Heat through for about 2 minutes.

Remove the skillet from heat and garnish the dish with freshly chopped parsley. Serve immediately.