Boil your pasta in a large pot with salted water and cook pasta 1 minute less than the box instructions
While the pasta is boiling, take a large pan and heat 2 tbsp of butter over medium heat. Once the butter has mostly melted, add in your onions and cook them just until they start to become translucent
Add in your garlic and cook for another 30 seconds or so followed by the remaining .5 tbsp of butter and allow it to melt
Add in your sliced mushrooms, salt, and pepper, and cook those down until they start to brown and start to reduce in size, around 5 minutes
Then add in your cajun seasoning and cook for another 2-3 minutes until the mushrooms and onions are evenly coated in the seasoning. Once the mushrooms are fully cooked down, deglaze your pot with the 1.5 cups of heavy cream and stir everything together and bring to a simmer
At this point, your pasta should be boiled. Reserve 3/4 cup of pasta water and add about 1/2 cup of the pasta water to the cajun mushroom sauce and continue to let it simmer for 2-3 more minutes
Drain the rest of the pasta and add it to the sauce and mix thoroughly so the sauce coats the pasta evenly. Add in the remaining 1/4 cup of pasta water to help the sauce stick to the pasta and cook on low for another 2 minutes. As it continues to cook the sauce will reduce slightly to give you the perfect consistency
Add your shredded pecorino romano cheese and fresh parsley, reserving some of both for garnish, and mix it into the pasta
Garnish with the remaining cheese, fresh parsley, and some pepper and serve hot!