Further research - website of the Gas Museum in Leicester states that "the forerunner ... of the modern gas cooker emerged in 1852... gas cooking became popular in the 1880s."

In Kipling's 'Stalky & Co' [1899], ch II, one of the characters uses metaphorically the words "He thought he had us on toast", meaning he thought he had us in a situation from which we could not escape — presumably by then an accepted metaphor for such a situation.

So would it be reasonable to suppose that the practice of serving savoury food 'on toast' is a late 19C culinary innovation? And does this cast any light, I wonder, on the earliest possible date of origin of this version of mine?