Put the mushrooms and milk into a saucepan, and heat until just boiling. Reduce the heat and simmer gently for 5 minutes.
Meanwhile toast the bread. Spread each slice with lower-fat spread.
Mix the cornflour with 2 tablespoons of cold water, then stir the mixture into the mushrooms. Heat until it thickens (this takes only a few seconds) and cook gently for 1 minute.
Divide the mushrooms evenly between the slices of toast and serve straight away.