There are a lot of different versions as to the origins of this dish and its rather odd name. The first recorded use of the name Welsh Rabbit dates from 1725. Welsh Rarebit a corrupted version of Rabbit, wasn't mentioned for another 60 years, when Francis Grose defined Welsh rabbit in A Classical Dictionary of the Vulgar Tongue in 1785, mistakenly indicating that rabbit was a corruption of rarebit."
In 1926, grammarian H. W. Fowler stated: "Welsh Rabbit is amusing and right. Welsh Rarebit is stupid and wrong."
Both names identify the same dish, usually consisting of toasted bread, topped with melted cheese, (usually cheddar) and mustard.
Hannah Glasse, author of the cookbook The Art of Cookery (first published in 1747) says:
During the 1950's, the dish, usually identified as "Welsh Rarebit" was rediscovered by American housewives, looking for simpler, easier to prepare, meals that could be make with new-fangled processed foods. Which is where today's recipe comes in.To make a Welch rabbit, toast the bread on both sides, then toast the cheese on one side, lay it on the toast, and with a hot iron brown the other side. You may rub it over with mustard.
From the Metropolitan Life Insurance Company's March 1957 edition of the Metropolitan Cook Book,
Tomato Welsh Rarebit
2 cups (1/2 lb.) Processed American Cheese, grated
1 10.5oz. can tomato soup (undiluted)
1 tbsp. minced onion
1 tbsp. ketchup
1/4 tsp. dry mustard
1/4 tsp. salt
dash of pepper
1 egg, beaten
Melt cheese in top of a double-boiler. Heat soup with all remaining ingredients, except egg; add to cheese. Add egg. Stir in double-boiler, for five minutes. Serve on crisp toast. Makes 4 servings.