Whenever you’re cooking, keep your vegetable clippings. Your onion tops, celery root, pepper tops and veins, potato peelings, etc. Store them in the freezer until you need some stock.
When you need stock, do this:
Put a bunch of the clippings in the pressure cooker and add 4 - 6 cups of water.
Cook it on high pressure for 15 minutes, then release the pressure. Pour the liquid through a strainer and collect it in some jars.
There! Now you have some tasty stock.
If you don’t have a pressure cooker, then use a slow cooker.
I usually pan fry some of the frozen vegetable clippings in my cast iron skillet for a while until they start to burn a little bit to get some toasty flavor, and then put it all in the slow cooker with plenty of water and let it cook on high over night. Then you can dish it up in the morning.