Hello my fried-egg-with-sauce Friend.

The Oxford dictionary defines "toast" as "a slice or piece of bread browned at the fire." Says it's "late Middle English" The same reference gives the phrase "on toast" without a date.

It would be reasonable to assume that food would have been added to the toast in the same way as it was added to bread.

The idea of toasting/grilling the food ON the toast could be achieved in any kind of oven -- like pizza. So as long as we've had ovens and bread I'm thinking we could have had "food on toast".

I'm not a gastro-historian though. Just someone who loves eating.
Cheers, Joy