Make up some couscous with boiling water (I use the plain couscous, never tried the flavoured ones, but I suppose you could use those). If the mushrooms have stalks, remove them carefully and chop them.
Mix the couscous with the chopped mushroom stalks, finely sliced spring onions, finely chopped courgette - any other vegetable you like, but it must be chopped small. Season to taste, add herbs if you like.
Put the mushrooms in a flat tin. Pile up the couscous mixture on each one and spread right to the edges. Sprinkle grated parmesan on the top and bake in the oven until brown and crispy on top.