Combine the ingredients in a stockpot large enough to hold them with about 6 inches of space at the top of the pot. Add cold water to cover by 1 inch.

Simmer the stock for 2 hours over medium heat. Strain through a fine-mesh sieve, pressing on the vegetables with the back of a large spoon to force out as much of the liquid as possible. Strain a second time through a fine-mesh sieve lined with 3 layers of dampened cheesecloth. Let cool. Use now, or cover and refrigerate for up to 1 week or freeze for up to 1 month.