First, get the equipment ready.
Next, get the ingredients.
Place the grated cheese into the bowl.
Add the mustard and milk, and then stir everything together.
Put the bread on a grill pan and, using oven gloves, place under a hot grill until the bread is toasted on one side.
Pull out the grill pan, remove the toast and place it on the plate. Turn it so that the un-toasted side is up.
Next, spoon the cheese mixture over and spread it right to the edges.
Carefully place the toast back under the hot grill and cook until the cheese bubbles.
Finally, remove the Welsh rarebit from the hot grill and serve.