Mayonnaise instead of butter for the outside of grilled cheese.
I had never heard of this until perusing Anthony Bourdain's cookbook (a birthday gift from my parents). I was immediately intrigued. Doubly so since he referenced Gabrielle Hamilton* as his introduction to this idea.
Since discovering this and talking to a number of people, apparently it's not as unique an idea as I surmised.
Either way I love it.
Because mayonnaise is oil-based (rather than fat-based) it slows the cooking time enough to even the cheese-melting/bread-frying ratio. Which means you don't have to meticulously hover over the pan to get a well-melted, well-fried piece of grilled cheese.**
There are other benefits, too. The sandwich tends to be softer (lower temp) and it tastes vaguely egg-y. And I don't terribly like eggs, but I still see this as a pro. That's how vague the egg-y taste is.
It's maybe the best cooking tip I've happened upon in quite some time.
*I own both Bourdain's and Hamilton's autobiographies. They represent about 1/3 of the autobiographies I own. Stephen Fry represents another third.
** I recognize that if you are using processed American cheese, this isn't an issue. I love me some processed grilled-cheese but processed American cheese is surprisingly expensive, especially for a single-person household.