In a medium-sized saucepan, boil water and salt it. Add your pasta, and cook according to the package instructions until al dente. When the pasta is cooked, reserve 1 cup of the pasta water and drain the pasta.
In a skillet over medium heat, melt the butter, and add olive oil. Add the sliced mushrooms, then season with salt, pepper, and cook until tender (about 3-4 minutes).
Add garlic, and cook for 30 seconds or until it's fragrant. Add the heavy cream, parmesan, and cajun seasoning.
Stir and cook until the parmesan melts then loosen the sauce with reserved pasta water and allow to simmer for a minute. Taste the sauce and see if more salt and pepper are needed.
Toss the pasta with the sauce until it's well coated. Add the prime rib to warm it up gently, garnish with freshly chopped parsley and serve.
