Fry the onion in low calorie cooking spray until softened
Add the chicken, and fry gently until no pink shows
Add the mushrooms and stir fry for a couple of minutes
Crumble in the 1/2 oxo cube and add the water, stir well, and season
Allow to bubble gently for about 15 minutes until the juice has thickened a little
Add the creme fraiche and simmer for a couple of minutes. If watery, allow to reduce slightly
Lay out the bottom halves of the sandwich thins onto a tray sprayed with low calorie cooking spray
Share the mixture between them, and use a pastry brush and some beaten egg to paint around the edges so that you can 'stick' on the top of the sandwich thin, press the edges of the tops down firmly to seal the slices.