Spray a frying pan with low calorie cooking spray and place over a medium heat.
Add the mushrooms and sauté for 5-6 minutes until soft.
Remove from the heat and add the water and Marmite. Stir well. The residual heat in the pan will allow the Marmite to dissolve.
Pop the bread in the toaster and return the pan to stove on a lower heat.
Stir in the Philadelphia Light until well combined and heated through.
Serve the mushrooms over the toast and sprinkle with chopped chives if desired.