Have I imagined this but I thought that when making rarebit, the sauce bit of it, is made up on the hob, then put in a small bread tin in the oven to bake and then when you put it on the bread, you slice it off and layer it on the bread and then put the bread under the grill
I know this isnt how its done now but when I first learned about this, there were recipes setting that process out but I cant find them now. Im talking about 30 years ago.