I've been working on making smaller size amounts of food, less leftovers with varying success. " Working on it" is the operative word.
Onions vary in size so much. And then there is the "we" factor. What a medium onion to one of us may be a large to another. It would be simpler if recipes specified use by Cup size amounts, but then, what is one to do with a little bit of left-over onion, toss it? or use it. Of course, Use it and it's a Heaping Size measure. So here we are back to the beginning.
I have seen bags of diced onions in the freezer section. I think that could be helpful when you want a specific amount, such as a handful. Now, there's an accurate measurement.
I look for bags of onions that are smaller in size ( when possible) so I can use just some or a little or add a 2nd smaller onion if more is wanted.