I very frequently put half onions back in the fridge. I have a set of silicon cover things that you can just put on the end which work really well. If you cut an avocado the “wrong” way so the cut end is round they also work for them - with the bonus that it is easy to cut into thirds or other amounts if you don’t want a half. I also have lots of little containers that get used for various cut bits and pieces. One of the perks of single live.