I just keep the "extras" of the veggies I use and then throw them in a pot w/ some garlic, peppercorns and a little bit of salt.
right now in my container in the fridge I have...
some collard green stalks
unusuable parts of tomatoes
ends of celery (both leaves and near the root part)
ends of turnips
bits of green onion and regular onion
ends of carrots
ends of zucchini
I think there are also some bits of turnip green stalks in the mix also right now. but I think you get the point. I just throw whatever I have in there, I usually collect the veggies for about a week. I even made a stock with some sweet potatoe bits once. I just cook it all up, then strain it and its done. I freeze it in ice cube trays and then keep it in freezer bags in the freezer and use it when I need it.
I think it is always good no matter what I put in it. Well, I have not had it turn out awful yet.
The best stock in the history of my world is whats left over from when I make seitan. I simmer the wheat gluten in a pot of water (for an hour) with a quartered onion, a few carrots chopped in half, a few stalks of celery cut to fit in the pot and a bit of tamari or braggs. When the seitan is cooked I strain the broth and put it in containers and freeze it. It's amazing.
1 Tbsp oil
2 onions, quartered
2 large carrots, cut into large pcs
1 celery rib, cut into large pcs
2 - 3 garlic cloves
peels from 2 large well-scrubbed potatoes
1 bay leaf
1/2 tsp black peppercorns
8 cups water
2 tsp soy sauce
1 tsp salt
Drizzle oil in the boottom of a slow cooker.
Add everything else on top. Cover and cook on low for 8 -10 hrs. Strain.