Step 1

Begin by laying the slices of bread on a baking tray, and placing them under the grill (broiler), set to a medium-high heat. Cook for a couple of minutes until one side of the bread is golden brown and crispy. Then, turn each slice over and remove from the grill.

Step 2

Meanwhile, melt the butter in a small saucepan, and add the flour. Mix well for 30 seconds, then add half of the milk. Stir well until the sauce is smooth, then add the remaining milk and mix again. The mixture should thicken up into a smooth, thick paste.

Step 3

Add the grated cheese, mustard, and black pepper. Cook for another minute or two, stirring constantly, until the cheese has melted and the mixture is smooth.

Step 4

Spoon the sauce onto the untoasted side of each piece of bread, gently pushing it to the edges with the back of the spoon (don't worry if it spills over the edges a bit!).

Step 5

Place back under the grill (broiler) for about 5 more minutes, until the cheese sauce is just beginning to brown. Serve immediately.