In a large skillet, heat half the butter along with all the olive oil over medium heat. Once the butter has melted, add the mushrooms and cook them, allowing the water to release and evaporate. Continue cooking until the mushrooms develop a golden edge, about 5 minutes. Midway through cooking, season with salt and pepper.
Next, add the minced garlic and the remaining butter, cooking for another 2 minutes until the garlic and mushrooms turn a lovely golden color.
Incorporate the cooked pasta into the skillet, along with about 3/4 cup of the reserved pasta water and the freshly grated parmesan. Toss gently until the liquid thickens into a glossy sauce that clings to the pasta. If the pasta seems dry, add more of the reserved pasta water as needed.
Taste, and adjust the seasoning with additional salt and pepper if necessary. Serve immediately, garnished with a sprinkle of fresh parsley and extra parmesan.