4 tablespoons butter
1 onion, roughly chopped
3/4 pound cremini mushrooms or dark wild mushrooms, chopped
Kosher salt and freshly ground black pepper
2 cloves garlic, minced
3 cups chicken stock
3/4 cup pistachio nuts, roughly chopped, plus additional for garnish
1 teaspoon chili flakes
3 tablespoons extra-virgin olive oil
1 pound your favorite pasta
Grated Parmesan or pecorino