One of the most important parts of doing "once a month cooking" is the prep work. Even if you don't do all of the cooking on one day, you'll want to do as much of the prep work at one time as possible. This way, even if you need to stop early, you have still shaved off tons of time for later in the week or month.
The above is an example of peeling a whole bunch of carrots at once. I ran out of time and couldn’t go ahead and slice/dice/chunk them for various recipes. I was also making enchiladas for dinner and had just emptied a tortilla. Instead of throwing it out, it was an easy, reusable way to store the whole, peeled carrots, until I was ready to come back to them again.
Later in the week, I likely sliced some of these for the kids’ lunches, diced some for soups, and polished off the last for snacks. Chances are, I had a similar bag of celery right alongside it in the fridge, too.