Meanwhile, melt the butter in a large skillet over medium-high heat. Add the onions and cook, stirring occasionally, until lightly browned, about 7 minutes. Add the mushrooms, season with salt and pepper, and cook, stirring occasionally, for 3 minutes. Stir in the garlic and cook for another 2 minutes, then reduce the heat to medium. Add the stock and simmer until the liquid is reduced by half. Stir in the pistachios, crushed red pepper and oil, and season with salt and pepper. Set aside and keep warm.
Meanwhile, add the pasta to the boiling water and cook according to the package directions. Drain, then serve right away with the sauce spooned over the top. Sprinkle with Parmesan and pistachios.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.