We make stock pretty often at our house. I used to get frustrated when I wanted to make stock but didn’t have any carrots, celery, or onion to throw in, until I learned this little trick!
Whenever you are using carrots, celery, or onion, save the ends and scraps and place them in a freezer bag in the freezer. Then, when you are ready to make stock, you can just take out the bag, dump the contents in with the bones, and return the bag to the freezer to be refilled in the coming weeks.
I find this particularly helpful for celery and onions, both of which we eat quite often, but since it is just the two of us right now, we often have lots of leftover scraps. My husband is not a huge onion fan so I rarely use a whole onion at once, and if my stash of frozen onions is full, then I will throw the scraps into my scrap bag for later use!
I have friends who also use the ‘waste’ from their juicer to add to stock for that extra flavor and nutritious punch.