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1
Preheat the broiler. Arrange the onions and ginger on a foil-lined baking sheet. Once the broiler is hot, broil the vegetables close to the heat source until charred all over, flipping them with tongs as needed. Remove the onions if they cook more quickly than the ginger, or vice versa.
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2
Alternatively, char the onions and ginger over the open flame of a gas burner, turning them periodically, until blackened all over. This will need to be done in a few batches.
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3
Heat the oil in a stockpot over medium-high heat. Once hot, add the leeks, carrots, daikon, garlic, lemongrass, star anise, cloves, cinnamon, and fennel seeds. Stir to coat in the oil, then cover and cook for 5 minutes, until fragrant and the colors of the vegetables are vibrant. Coarsely chop the charred ginger, then add it, along with the onions and the mushrooms, to the pot and cover with cold water; you'll need about 4 quarts. Bring to a boil, then reduce to a gentle simmer and cook for 1 hour, at which point the broth should be strongly flavored. Add the cilantro stems and cook for another 5 minutes.
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4
Strain the broth through a cheesecloth-lined sieve, in batches as necessary, gathering up the ends of the cheesecloth so as to squeeze out as much liquid as possible. Once completely cooled, pack in containers and store in the refrigerator for up to 1 day, or in the freezer for up to 2 months.
