Preheat the grill to 250'C.
Peel and finely dice the. Grate the cheese. Toast the bread on both sides.
In a small saucepan melt the butter and cook the diced onion over a low heat until completely soft. Add the flour and continue cooking until the butter and flour is well combined and you have a light coloured roux.
Warm the Guinness and add this to your roux. Keep cooking and stirring until you have a thick and smooth sauce. Taste the sauce at this stage to check that the flour is cooked.
Add the grated cheese and stir until the cheese is completely melted. Mix in the mustard and Worcestershire sauce and check the seasoning.
Spread the mixture on the toasts. Cook under the hot grill for a few minutes until the cheese is browned and bubbling.
«Add some spice to the rarebit by adding some Tabasco sauce or finely diced chillies.»