Step 1: Check the cupboard
I also had an onion and a shallot! (I later replanted the bottom of the shallot to grow a new one!)
You'll want some kind of oil to saute your onions
If you have vegetable stock or some such thing, awesome for you! You can just use water.
I was lucky enough to find a bouillon cube tucked away in the cupboard!
If you've got more stuff you want to add in - that celery wilting in the bottom drawer, some of those baby carrots in the back of the fridge - whatever you find, put it in there!