Vegetable stock is delicious, but it's expensive to buy and a pain to make. The idea of keeping vegetable scraps to make it from (saving hassle and expense) is an old one, but the scraps always spoiled before I had enough to make a balanced broth. But a few years ago, a friend of ours mentioned that he just keeps the scraps in the freezer until he has enough. Brilliant! We now keep a large tub in the freezer and wait until it's full to make vegetable stock. Using a pressure cooker it only takes about 35 minutes or so to cook (we cook it for 25 minutes on high after it's up to pressure).
Here's what it looks like before cooking:
In this batch we had leek greens, parsnip and carrot peels, red and yellow tomato ends, mushroom stems, and garlic ends. Other times we put in celery leaves, sweet potato or butternut squash peels, sometimes even kale stems for health (although inexplicably our dog loves kale stems, so she usually gets those now). Things that are slightly blemished are fine to go in (you'll filter out the solids anyway) even if you wouldn't put them in soup. Just nothing that has spoiled. Also, don't put too much onion / garlic or they can dominate the flavor.
dump the big bowl of frozen scraps into a pressure cooker
If you don't plan to compost it in a worm bin afterwards, add some salt (maybe 1/2 tsp) which will help to pull the flavors out
cook on high 25 minutes
strain out into a big glass bowl, use some in a recipe right away if you like (or put in the fridge if you'll use it within a few days). Break the rest up into 2 cup increments and store in the freezer for whenever you need some vegetable stock
We make ours salt-free because afterwards the cooked scraps go into our worm bin. So we get remarkably good fresh veggie broth for free (and by freezing it, we always have some available), and nothing gets wasted since it all gets composted in the end and was just scraps that we wouldn't use otherwise anyway! This is truly one of our favorite kitchen tricks.