Preheat oven to 400°F.
Place prepared shrimp in a large heat proof bowl.
Prepare pasta according to package directions. Drain. Immediately add hot pasta to bowl with shrimp. The heat will partially cook the shrimp.
Add 2 tablespoons butter to a medium sauce pan. Set over medium heat and melt butter.
Add onion. Sauté 1 minute.
Whisk in flour. Cook, stirring with a whisk for 1 minute.
Whisk in milk. Heat approximately 4 minutes.
Add Cheddar and Monterey Jack cheeses. Stir until smooth.
Add 1/2 teaspoons Cajun seasoning, salt and pepper.
Remove from heat and stir in shrimp and pasta. Mix well.
Transfer the mixture to a buttered casserole dish.
Top with Parmesan cheese, bread crumbs, 3 tablespoons melted butter and parsley.
Place in preheated oven. Bake 20 - 25 minutes or until browned and bubbling.
Serve hot.