In a large pot of salted boiling water, cook the fettuccine according to package instructions until al dente. Drain and set aside, reserving ½ cup of the pasta cooking water.
In a bowl, toss the cleaned shrimp with Cajun seasoning until evenly coated.
In a large skillet over medium-high heat, melt 2 tablespoons of butter. Add the seasoned shrimp and cook for 2-3 minutes on each side until they are pink and opaque. Remove the shrimp from the skillet and set aside.
In the same skillet, add the remaining tablespoon of butter and minced garlic. Sauté for about 1 minute until fragrant.
Pour in the heavy cream and bring to a gentle simmer. Gradually stir in the grated Parmesan cheese until melted and the sauce is smooth. If the sauce is too thick, add a bit of the reserved pasta water until you reach your desired creaminess.
Add the cooked fettuccine and sautéed shrimp to the Alfredo sauce. Toss gently to combine, ensuring the pasta and shrimp are well coated. Season with salt and pepper to taste.
Divide the Cajun Shrimp Alfredo into serving bowls and garnish with chopped parsley and a squeeze of fresh lemon juice. Serve immediately with lemon wedges on the side.