Put the cheddar and beer in a pan and heat gently until the cheese melts and starts to bubble. Add the flour and cook for a minute then remove from the heat. Add the breadcrumbs, mustard, egg yolks and season. Mix well and allow to cool. Grill the bread on both sides and put on a plate. Top both slices with the Rarebit mixture and place under the grill to brown. Serve immediately.
The rarebit mixture can also be used to top a piece of pollock or cod. Bake in a hot oven for 15-18 mins and serve with roasted baby vine tomatoes.