Bring a large pot of well-salted water to a boil. Cook rotini until al dente per package directions. Reserve 1/2 cup pasta water, then drain and set pasta aside.

Meanwhile, heat olive oil in a large skillet over medium-high heat. Add ground beef and cook, breaking into crumbles, until browned with caramelized edges; drain excess fat if needed.

Push beef to one side. Add diced onion to the open side and cook 2–3 minutes until softened. Stir in minced garlic and cook 30 seconds until fragrant.

Sprinkle Cajun seasoning (and red pepper flakes if using) over the skillet. Stir and toast 1 minute to bloom the spices.

Reduce heat to medium. Pour in heavy cream and beef broth, scraping up any browned bits. Simmer gently 5–7 minutes until the sauce thickens slightly.

Add cooked rotini to the skillet and toss to coat. Stir in grated Parmesan until melted and the sauce clings to the pasta.

Adjust consistency with splashes of reserved pasta water as needed. Season to taste with salt and black pepper.

Remove from heat, garnish with chopped parsley or green onions, and serve warm.