2 onions, roughly cut (leave the skin on for extra flavor and color)
2 leeks, roughly cut
3 garlic cloves, crushed
3 carrots, roughly cut
4 celery stalks, roughly cut
a little lemon zest
a handful of fresh parsley, or 1 1/2 teaspoon dried
some fresh thyme
1/2 teaspoon freshly cracked pepper
2 bay leaves
4 whole allspice
8-10 cups water
Vary the veggies and spices according to whim and availability. I highly recommend using fresh herbs for this as dried tend to get scooped out with the foam. If using fresh herbs, tie together in a bit of cheesecloth.
Throw everything in a large stock pot, bring to a rapid boil. Skim off the foam, reduce the heat and simmer for 30 minutes or so, skimming the foam when it rises.
Strain the stock let it cool.
An alternative: I like to bring to a second boil and reduce stock by half, this cuts down on storage space as I like to make huge amounts of stock at a time. Freeze, and when you're ready to use, dilute with an equal amount of water.