Cook the pasta according to package directions until al dente. Reserve about 1 cup of pasta water before draining.

While the pasta is cooking, season the shrimp with salt, pepper, and half of the Cajun seasoning.

In a large skillet or Dutch oven, melt 2 tablespoons of butter over medium-high heat. Add the shrimp and cook for 2-3 minutes per side, until pink and cooked through. Remove the shrimp from the skillet and set aside.

Add the remaining 2 tablespoons of butter to the skillet. Add the onion and bell pepper and cook until softened, about 5 minutes. Add the garlic, smoked paprika, cayenne pepper (if using), and remaining Cajun seasoning and cook for 1 minute more, until fragrant.

Pour in the white wine (or chicken broth) and scrape up any browned bits from the bottom of the skillet. Let the wine reduce by half, about 2-3 minutes.

Stir in the heavy cream and bring to a simmer. Reduce the heat to low and let the sauce thicken slightly, about 5 minutes.

Stir in the Parmesan cheese until melted and smooth. Season the sauce with salt and pepper to taste.

Gently fold in the cooked shrimp and crab meat. Heat through.

Add the cooked pasta to the sauce and toss to coat. If the sauce is too thick, add a little of the reserved pasta water until it reaches your desired consistency.

Serve immediately, garnished with fresh parsley and extra Parmesan cheese. Offer lemon wedges on the side, if desired.