Melt butter in small saucepan, add flour stirring until well combined. Add mustard & new vegemite. Using a whisk slowly add the milk; as mixture thickens add 1/2 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 oven tray.
Spread each slice of toast generously with rarebit mixture. Top with remaining cheese & season to taste. Place under low grill 5 minutes or until cheese is golden.