Melt butter in small saucepan, add flour stirring until well combined. Add mustard and Vegemite. Using a whisk, slowly add the milk; as mixture thickens add 1/2 of the cheese. Mix over low heat until mixture is thick and cheese is melted through.
Remove from heat, allow to cool 5-10 minutes.
Meanwhile toast your bread and place on baking tray.
Spread each slice of toast generously with cheese mixture. Top with remaining cheese and season to taste. Place under a low grill for 5 minutes or until cheese is golden.