Bring a medium saucepan of water to a boil and salt it generously.
Cook the pasta according to package instructions until al dente.
Reserve 1 cup of pasta water and drain the pasta.
In a skillet over medium heat, melt butter with olive oil.
Add sliced mushrooms, season with salt and pepper, and cook 3 to 4 minutes until tender.
Stir in minced garlic and cook for 30 seconds until fragrant.
Pour in heavy cream, add Parmesan cheese and Cajun seasoning, stirring until cheese melts and sauce is smooth.
Loosen sauce with reserved pasta water and simmer for 1 minute, adjusting seasoning if needed.
Toss cooked pasta in the sauce until evenly coated.
Gently fold in prime rib pieces to warm through.
Garnish with chopped parsley and serve immediately.