Bring a large pot of salted water to a boil. Cook the fettuccine until al dente according to the package directions. Before draining, reserve 1/2 cup pasta water.

Pat the shrimp dry and toss with the Cajun seasoning. Heat the olive oil in a large skillet over medium-high heat. Cook the shrimp in a single layer for about 1 1/2 to 2 minutes per side, working in batches if needed, until opaque and cooked through. Transfer to a plate.

Reduce the heat to medium. Add the butter and garlic to the skillet and cook for about 30 seconds, stirring, just until fragrant. Pour in the cream and bring it to a gentle simmer for 2 to 3 minutes; do not boil hard. Stir in the Parmesan until smooth.

Add the drained pasta and toss to coat, loosening the sauce with pasta water a tablespoon or two at a time. Return the shrimp to the skillet and warm briefly. Finish with parsley, lemon juice, and black pepper. Taste before adding salt because Cajun seasoning and Parmesan can be salty.