How does this work? I've seen it recommended before and can't get my head around why it works.

Acidity mixed with heat

Onion works cause it's dense & acidic. When I worked with a flat top griddle way back,we used lemon juice because nothing short of actual aggressive de-greasing chemicals came close to how good it worked overall and without the dangerous fumes. Could probably soak a towel in lemon juice for a grill

When I worked at a burger joint, we'd clean the grill with water. The water on the hot grill would boil, and de-glaze the grill surface. Finish that up with scotch bright pad, and it was good to go.

We'd do the same in a rush if necessary, end of shift was always lemon juice, it went quite a bit farther. I did work at a higher end private restaurant though, so things were a bit more picky than most fast food places I've had experience in.

The acidic liquid in the onion, combined with the hot grill grate, creates steam and helps remove built up material on the grill grate. I think the relative weight/density of an average onion is of some benefit as well.

onions have an array of chemical compounds including sulfuric acid. The acids help break down the organic buildup on the grill. So while a brush is using mechanical abrasion to knock off the organic buildup, an onion will chemically break down bonds, including the bonds to your grill

My buddy is in remediation and is a chemical whiz with cleaning. I followed up with him and he said the onion is really cool because there are multiple chemical reactions happening from the acid and other compounds.

The water in the onion is turned to steam from the heat which helps it permeate the char and increases the rate of reactivity. The acids meanwhile are dissolving some of the simple organic bonds The steam and acids actually cause hydrolysis which breaks down more complex oranges bonds

apparently onions are awesome and I'm never going to hear the end of it. So now you must all suffer in kind.

he called me to talk about cleaning vinegar 😂 I want to blame yall, but I knew this would happen

Hahahaha! That's awesome.

lol I always love hearing from him. Even if it's painfully detailed information about cleaning.

It'll make your grill taste good, too! Win win!

Half a lemon as well esp if used up first.

Either adds flavour to the bars! Lemony meats mmmmm