Lemon, although I realise it's not quite a cupboard thing.
I would be lost without tumeric, cumin, chilli powder and corriander.
Absolutely must have 2-3 onions and a couple of cloves of garlic around as well.
I don't think I could choose one thing, stuff like garlic, lemongrass, ginger etc. all seem as obvious as salt and pepper.
Have you tried preserving them? They last for ages if you do that and are a good addition to numerous dishes.
On another lemon note, I went to Turkey recently and ended up buying a bag of lemon salt. It looks like salt crystals, but tastes like lemon! Amazing stuff.
One thing?
Salt I reckon
I am not talking about salt and pepper and basic stuff, but one thing that you have to have in your kitchen cupboard?
Can be a cooking item or a foodstuff.
You should be able to make your own by putting lemon wedges in a jar full of sea salt crystals and waiting a few months.Lemon salt sounds amazing.