Bring a large pot of salted water to a boil. Add rigatoni and cook until al dente according to package instructions. Reserve ½ cup pasta water, drain, and set aside.
In a saucepan, melt 2 tbsp butter over medium heat. Add half of the minced garlic and sauté for 30 seconds until fragrant. Stir in heavy cream and milk until smooth. Lower heat and whisk in cheddar and Parmesan until melted and creamy. Season with Italian seasoning, salt, pepper, and red pepper flakes if using. Toss rigatoni into the sauce until fully coated, adding a splash of pasta water if needed for creaminess.
Pat steak cubes dry with paper towels. Season generously with Cajun seasoning, smoked paprika, salt, and pepper. Heat olive oil in a cast-iron skillet over medium-high heat. Sear steak tips in a single layer for 2–3 minutes per side until browned and cooked to desired doneness.
Reduce heat slightly, add 3 tbsp butter and the remaining garlic, and baste the steak tips in garlic butter for 1–2 minutes, spooning the butter over the steak for flavor.
Serve with the cheesy rigatoni on one side and the garlic butter steak tips on the other. Drizzle extra garlic butter over the steak and garnish everything with parsley.
