Fish paste. Specifically Sardine and Tomato, in little jars. We had it in sandwiches when I was growing up. I thought they'd stopped making it but it's still available. I know it's probably a by-product of the sardine-canning industry and I'm not going to think too deeply about what's in it, but I find myself craving a slice of buttered toast with that ultimate umami, salty, fishy flavour. Mmmmmm. Wonder if they sell it at the corner shop?
And while I'm getting nostalgic, do they still make Heinz sandwich spread?