Cook the pasta: Bring a large pot of salted water to a boil. Cook pasta until just al dente. Reserve about 1/2 cup pasta water, then drain.
Season the shrimp: Pat shrimp dry.
Toss with 1 tablespoon Cajun seasoning, a pinch of salt, and the smoked paprika.
Sear the shrimp: Heat olive oil in a large skillet over medium-high. Add shrimp in a single layer and cook 1–2 minutes per side until opaque and lightly browned. Transfer to a plate.
Build the base: Reduce heat to medium.
Add butter to the same skillet. Sauté onion and bell pepper with a pinch of salt for 3–4 minutes until softened. Stir in garlic for 30 seconds.
Deglaze: Pour in white wine or broth.
Scrape up any browned bits and simmer for 1–2 minutes to reduce slightly.
Make it creamy: Stir in heavy cream and the remaining 1 tablespoon Cajun seasoning. Simmer gently for 2–3 minutes until slightly thickened.
Add cheese and balance: Stir in Parmesan until melted and smooth. Add lemon juice.
Taste and adjust salt, pepper, and heat with red pepper flakes if needed.
Toss with pasta: Add cooked pasta to the skillet and toss to coat. If the sauce is too thick, loosen with splashes of reserved pasta water until silky.
Finish with shrimp: Return shrimp to the pan and toss for 1 minute to warm through. Garnish with parsley and serve hot.