Bring a large pot of cautiously salted water to a boil. Cook the pasta according to the package directions until al dente, reserve about 1/2 cup of the starchy pasta water, and drain.

Pat the shrimp dry, then toss them with about 1/2 tablespoon of the Cajun seasoning and the black pepper. Dry shrimp sear more effectively and help prevent excess liquid from thinning the sauce.

Heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Cook the shrimp for about 1 to 2 minutes per side, just until pink and opaque, then transfer them to a plate.

Reduce the skillet heat to medium and add the remaining butter, onion, and bell pepper. Cook for 3 to 4 minutes until softened, then stir in the garlic and cook for about 30 seconds.

Add the remaining Cajun seasoning and optional paprika to the skillet. Pour in the heavy cream and bring it to a gentle simmer; avoid a rapid boil.

Simmer the cream for 2 to 4 minutes, then gradually stir in the Parmesan until melted. Add reserved pasta water a tablespoon or two at a time if the sauce becomes too thick.

Add the drained pasta and cooked shrimp to the skillet. Toss for about 1 to 2 minutes until the pasta is coated and the shrimp are heated through.

Finish with chopped parsley or green onions and optional lemon juice. Taste before adding salt because Cajun seasoning, Parmesan, and salted pasta water may already provide enough seasoning.