Heat the butter or oil over medium-low heat in a large stockpot. Add the remaining ingredients and cook uncovered, stirring frequently, until they have softened and released their juices, about 30 min. (don’t let them brown). Add enough cold water to the pot to just cover the vegetables, about 4 cups. Bring to a gentle simmer, cover, and cook without stirring until the stock is flavorful, about 45 min. (adjust the heat as needed to maintain a gentle simmer). Strain the stock immediately through a fine sieve, pressing gently on the vegetables. Let cool to room temperature and then store in the refrigerator for up to a week, or freeze for up to six months.
Photo: Scott Phillips