01 - Cook pasta in a large pot of salted boiling water according to package directions. Drain and set aside, reserving 1/2 cup of pasta cooking water.
02 - Pat shrimp completely dry with paper towels. Toss with 1 tablespoon Cajun seasoning and a pinch of salt and pepper until evenly coated.
03 - Heat 1 tablespoon butter in a large skillet over medium-high heat. Arrange shrimp in a single layer and cook for 2 minutes per side until just opaque and pink. Remove from skillet and set aside.
04 - Add remaining butter to the skillet. Add sliced onions and all bell peppers. Sauté for 4 to 5 minutes until vegetables are softened and edges begin to caramelize.
05 - Stir in garlic, remaining Cajun seasoning, smoked paprika, and cayenne if using. Cook for 1 minute until fragrant, stirring constantly to prevent burning.
06 - Reduce heat to medium. Pour in heavy cream while stirring to combine with spices. Simmer for 2 to 3 minutes until sauce slightly thickens and coats the back of a spoon.
07 - Stir in Parmesan cheese until melted and smooth. Add drained pasta to the skillet and toss thoroughly to coat. If sauce becomes too thick, add reserved pasta water a tablespoon at a time until desired consistency.
08 - Return seared shrimp to the skillet. Toss gently to combine and heat through for 1 to 2 minutes. Taste and adjust seasoning with additional salt and pepper as needed.
09 - Plate immediately while hot. Garnish generously with chopped fresh parsley, additional Cajun seasoning, and lemon wedges on the side.