I would use laughing cow lights or extra lights (as part of your HEa).
mash them with some crushed garlic (fresh) and fresh chopped parsley and then add it to the top of your mushrooms.
If you have a HEb spare, blitz your wholemeal bread into some bread crumbs and sprinkle on top and then spray with a bit of frylight and bake in the oven or you could just syn a slice of wholewheat low caloried bread for the breadcrumbs, as it won't be many syns.
season the top with some black pepper and salt if needed.
If you eat meat or are doing an extra easy day/original day you could also add some chopped up ham or prosciutto to the cheese mixture.