Spray a large deep pan (I used my favourite Chef’s pan) with olive oil spray. Add the sliced onions and mushrooms and cook over low heat, stirring often, until softened – about 10 minutes.
Preheat the grill (broiler)to medium-high. Transfer to a baking dish (or you can use the pan if it is oven safe). If it looking a little dry you can stir in some of the reserved pasta water or a little beef stock. Sprinkle with the grated Parmesan and add freshly ground pepper.