Bring a large pot of salted water to a boil and cook rigatoni according to package instructions until al dente.
Reserve 1/2 cup of pasta water, then drain the rigatoni and set aside.
Pat steak cubes dry with paper towels and season thoroughly with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat until shimmering.
Sear steak bites in batches for 2 minutes per side until a brown crust forms, then remove from skillet and set aside.
Reduce heat to medium and add butter to the same skillet, scraping up browned bits.
Add minced garlic and red pepper flakes, sautéing for 1 minute until fragrant.
Whisk in heavy cream and reserved pasta water, bringing to a light simmer.
Stir in mozzarella and parmesan cheeses until fully melted and smooth.
Return the steak bites and cooked rigatoni to the skillet, tossing to coat thoroughly in the cheese sauce.
Garnish with fresh parsley and serve immediately.