Step 1Bring the broth and 6 cups water to a boil in a large saucepan over high heat. While the broth is coming to a boil, turn another burner to medium-high if using a gas stove (if using an electric or induction stovetop, heat a cast iron skillet over medium-high heat). Put the onion directly on the grate over a gas flame or in the skillet. Cook, turning occasionally, until evenly charred all over, about 5 minutes. Transfer the onion to the broth.
Step 2Once the broth comes to a boil, reduce the heat to maintain a low simmer. Add the ginger, kombu, cinnamon stick, peppercorns, fennel seeds, coriander seeds and saffron. Simmer for 30 minutes. Strain the broth through a fine-mesh sieve into a large saucepan or into airtight containers to store until ready to use.