First, preheat oven to 375°F (190°C).
Now, take a large pot and cook elbow pasta according to package instructions until al dente, and drain.
In a large skillet heat 2 sticks of butter on medium heat. Add shrimp, cajun seasoning and cook until shrimp is cooked through. Then, remove the shrimp from the skillet, and set them aside.
Now, melt the remaining 2 sticks butter over medium heat. Add onion and cook for 2 minutes, stirring occasionally.
Then, add Cheddar and Monterey Jack cheese to the saucepan and whisk over low heat until both cheeses are melted.
Now, add flour to 1/2 glass of water. Whisk and stir constantly so that there is no lump. Then, add the flour mixture to the pan where you've melted cheese and mix thoroughly. It will make the sauce thick and heavy.
Add salt and pepper to taste.
Now, it's time to bake. Add cooked pasta to a 9x13-inch baking dish. Pour cheese sauce over top and stir until evenly combined. Sprinkle Parmesan cheese on top followed by breadcrumbs and parsley.
Bake for 15-20 minutes or until the top is golden brown and bubbly. Serve immediately.