In a nonstick skillet on medium heat, melt butter and sauté onions, mushrooms and bell peppers until soft, about 2-3 minutes
Once soft and fragrant season with salt and pepper and add garlic, heavy cream and parmesan cheese, mix well until everything is well combined
Next, season with cajun seasoning to taste and lower heat to medium low
Allow sauce to come to a simmer and thicken, about 5 minutes
Sauce is done when you can trace a line across the back of the spoon with your fingertip. If the line remains visible, the sauce is thick enough
Add cooked pasta to the sauce and combine well until all the noodles are well coated
Once pasta and sauce is combined, turned off the heat and add shrimp on top
Top with freshly chopped parsley if desired and serve hot