Boil 12 oz of rotini in salted water until al dente (about 8-10 minutes). Reserve 1/4 cup of pasta water, then drain.
Toss 2 lbs of bite-sized sirloin steak pieces with 2 tbsp Cajun seasoning until evenly coated. Let sit for 5 minutes.
Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium-high heat. Sear steak in batches for 2-3 minutes per side until browned. Remove and set aside.
In the same skillet, sauté 4 minced garlic cloves for 30 seconds until fragrant. Pour in 1/2 cup chicken broth and 1/2 cup heavy cream, stirring to combine. Simmer for 2 minutes.
Stir in 1 cup mozzarella and 1/2 cup Parmesan until melted and smooth. Add reserved pasta water if needed to thin the sauce.
Return steak to the skillet, add cooked rotini, and toss to coat in the cheesy sauce. Season with salt and pepper to taste.
Sprinkle with chopped parsley for garnish. Serve hot.
Notes
Adjust Cajun seasoning to control spice level. For a lighter version, swap steak for chicken or use low-fat cream. Store leftovers in an airtight container in the fridge for up to 3 days or freeze for up to 1 month.