I have long been ambitious to make a Welsh rarebit-type cheese on toast without seeing the mixture ooze over the sides of the bread and congeal at the bottom of the grill pan. The answer, as it is with potato cakes, seems to be an egg.
80g Cheddar or Gruyere
1/2tsp Dijon mustard
A few splashes of Worcester sauce
Scraping of nutmeg
1 egg, beaten
Pepper (you shouldn't need salt, because of the salty cheese and sauce)
4 slices bread
Mash together the ingredients (except for the bread).
You don't want the toast to burn before the cheese softens and bubbles, so set the grill to medium/low. Lightly toast one side of the bread. Turn over the slices, and toast just long enough to begin crisping the bread. Remove, and spread with butter and then the cheese mixture. Return to the grill until the cheese has browned on top.