buffy said:

OK, so when I was unloading the chest freezer for defrosting yesterday, I put a couple of kg of frozen apple slices into a big pot with some water, turned on the gas to boil them…and forgot until I smelt the burning. So now one of my good stainless steel pots is burnt. I’ve scraped the mess into the compost and I’ve attended to the outside with Jex and Ajax. Although Gumption paste worked a lot better than Ajax. Now I need to clean the inside, which is more badly burnt on. The internets suggest boiling vinegar and water, which I am now doing, to loosen the black stuff. Anyone here got any other ideas? I was thinking I might cut up some rhubarb leaves and boil them up (separate boiling from the vinegar one) and see if that helps. Every time I cook rhubarb my saucepan comes out lovely and shiny. I presume that is the oxalic acid.

Apparently is it onion to clean stainless steel?
maybe

https://www.homesandgardens.com/gardens/household-methods-to-clean-a-grill

Instead, Lindsey recommends using half a lemon. The method is identical: just substitute the onion half with half a lemon. She told me that ‘lemon juice contains acid that breaks down grease and stains. Rubbing lemon or lime halves over the grates helps remove gunk. The natural degreasing action of citrus can lift burnt spots when combined with some elbow grease and scrubbing.’

If you still need a little extra scrubbing, try dipping the lemon half in salt before you clean the grill. According to Lindsey, ‘coarse salt or dry rubs make for great grill-cleaning scrubs. Salt crystals won’t scratch the grill surface while removing layers of cooked-on messes. Any excess salt gets burned off next time you fire up the grill.’
Other methods for cleaning your grill

There are a couple of other home remedies worth considering if you’re taking on a dirty grill. Lindsey recommends an equal-part mix of water and vinegar, which is great for removing the truly burnt-on carbonization. We explore more ways to clean with vinegar in our dedicated feature.