Pat steak cubes completely dry with paper towels. Season all sides with Cajun seasoning, salt, and pepper. Heat 1 tablespoon butter and olive oil in a heavy skillet over medium-high heat until butter foams and subsides. Add steak in a single layer without crowding and sear for 2 to 3 minutes per side until deeply browned. Remove and set aside uncovered.

Bring a large pot of well-salted water to a boil. Cook rigatoni until al dente, about 10 to 12 minutes. Before draining, scoop out 1/2 cup pasta water and set aside in a small bowl. Drain pasta.

In the same skillet over medium heat, melt remaining 1 tablespoon butter. Add minced garlic and cook for 60 to 90 seconds, stirring constantly, until fragrant and just turning golden at the edges. Pour in heavy cream and bring to a gentle simmer, scraping up browned bits from the pan.

Reduce heat to low. Remove skillet from burner for 30 seconds then stir in Parmesan off direct heat until fully melted and smooth. Return to low heat. Add mozzarella or Fontina if using. Season with salt, pepper, and extra Cajun seasoning to taste.

Add drained rigatoni to the sauce and toss to coat. Add reserved pasta water a splash at a time to reach desired consistency. Gently fold in steak tips to warm through. Plate immediately topped with extra Parmesan and cracked black pepper.