Step 1: Season the steak tips generously with Cajun seasoning. Toss to ensure all pieces are coated evenly.
Step 2: Cook rigatoni according to package directions until al dente. Reserve 1 cup of pasta water before draining and setting aside.
Step 3: While pasta cooks, heat olive oil in a large skillet or Dutch oven over medium-high heat. Add seasoned steak tips and sear for 2-3 minutes per side until browned and cooked to your desired doneness. Remove steak from skillet and set aside on a plate.
Step 4: In the same skillet, melt butter over medium heat. Add diced onion and sauté until softened and translucent, about 3-5 minutes. Add minced garlic and cook for another minute until fragrant, being careful not to burn.
Step 5: Pour in chicken broth, scraping up any browned bits from the bottom of the pan to deglaze. Bring to a gentle simmer, then stir in heavy cream. Let it gently simmer for 5-7 minutes, stirring occasionally, allowing it to thicken slightly.
Step 6: Reduce heat to low. Gradually stir in the freshly grated Parmesan cheese, a handful at a time, continuously stirring until it melts into a smooth, creamy sauce. If the sauce is too thick, add a splash of the reserved pasta water until it reaches your desired consistency. Season with salt and pepper to taste.
Step 7: Add the cooked rigatoni and seared steak tips back into the Parmesan sauce. Toss gently to coat everything evenly, ensuring every noodle and steak tip is bathed in the creamy sauce.
Step 8: Serve immediately, garnished generously with fresh chopped parsley and optional red pepper flakes for an extra touch of color and heat.