batch cooking 'sauteed onions"- aka frozen onions.

This is one of the many things you can make when you're already firing up the oven so you aren't wasting fuel which is of course both money and very likely to be a non renewable resource. Freeze or store in fridge for later use.Most of my soups for  will start with 1 sauteed onion..that's about 1/2 cup.
Slice 5 onions into rounds. Throw in a baking vessel. Toss to coat in 3 tb olive oil. Bake at 350-425 (it will tolerate anything in these ranges) for 20-45 minutes (higher heat equals less time) Check and stir every 10 minutes.