Preheat the oven to 350 degrees F. Cook the elbow macaroni according to package directions. Drain and set aside.
Cut each shrimp into about 3 equal pieces. Melt 3 tablespoons of butter in a large skillet or medium-sized saucepan. Add the shrimp and sprinkle with ½ teaspoon of the Cajun seasoning. Cook until the shrimp starts to turn pink (they will finish cooking in the oven), about 3-4 minutes. Remove the shrimp from the pan and set aside.
In the same skillet, melt 2 tablespoons of butter, add the dried onion and cook over medium heat for about 1 minute. Whisk in the flour and cook for about 2 minutes more while whisking. Stir in the milk, a little at a time, while whisking. Continue cooking until the mixture is hot but not boiling, stirring frequently. Mix in the cheddar and Monterey jack cheese, mixing until smooth and the cheese has melted. Season with salt and pepper to taste. Fold in the shrimp and macaroni.
Pour the mixture into a lightly greased 9x13inch baking dish. In a small bowl mix together 3 tablespoons of melted butter, the parmesan cheese, breadcrumbs, parsley and remaining ½ teaspoon of Cajun seasoning. Sprinkle the mixture over the top of the macaroni and cheese.
Place into the oven and bake for 45 minutes until hot and bubbly and the top is golden brown.