Chop the beet leaves so they will fit into the pot.
In a medium stockpot, heat the oil over medium-low heat and add the onions; cook until caramelized, about 15-25 minutes.
Add the remaining ingredients and bring to boil, then reduce heat and simmer uncovered for two hours.
Remove from heat, cool, and strain the stock through a fine sieve or a cheesecloth-lined strainer, pressing on the vegetables to extract the juices. Discard the vegetables.
The stock can be refrigerated for 3-4 days or frozen for up to 3 months.
Notes
This recipe is a guide, feel free to get creative and use your favorite vegetables.