Melt the butter in a heavy sauce pan. stir in cheese, salt, pepper and mustard. Stir over a gentle heat until cheese melts. The beer should be stirred in last. Spread over the toast slices and then brown in the oven on broil setting until cheese browns.
Variation: Buck Rarebit
Buck Rarebit is Welsh Rarebit with a poached egg served on top of each slice.
~This is a wonderful side dish. I tried it without the beer and it was not nearly as good. The beer does something special to the cheese.