Cook the pasta. Bring a large pot of salted water to a boil.
Cook the pasta until just al dente. Reserve 1/2 cup of pasta water, then drain.
Season the shrimp. Pat the shrimp dry. Toss with 1 tablespoon Cajun seasoning and a pinch of salt and pepper.
Set aside.
Sear the shrimp. Heat olive oil in a large skillet over medium-high. Add shrimp in a single layer. Cook 1–2 minutes per side until just pink and curled.
Transfer to a plate.
Sauté aromatics. Lower heat to medium. Add butter to the same pan. Stir in onion and red bell pepper.
Cook 3–4 minutes until softened. Add garlic and cook 30 seconds until fragrant.
Build the sauce. Stir in the remaining 1 tablespoon Cajun seasoning, smoked paprika, and red pepper flakes. Add chicken broth and simmer 2 minutes, scraping up browned bits.
Make it creamy. Pour in heavy cream and bring to a gentle simmer.
Stir in Parmesan until melted and smooth. Taste and adjust salt and pepper.
Toss with pasta. Add drained pasta to the sauce. Toss to coat, splashing in reserved pasta water as needed to loosen and gloss the sauce.
Return the shrimp. Add shrimp back to the pan and warm through for 1 minute.
Squeeze in lemon juice and stir.
Finish and serve. Sprinkle with chopped parsley. Serve hot with extra lemon wedges and more Parmesan if you like.