Step 1 Peel then slice the ginger into 4 or 5 coins. Smack with the flat side of a knife or meat mallet and set aside. Thinly slice the green parts of the green onion to yield 2 to 3 tablespoons, and set aside to use as a garnish in the final dish. Cut the leftover sections into 2- to 3-inch lengths, bruise, then add to the ginger.
Step 2 Coarsely chop the leafy tops of the cilantro to yield 2 tablespoons; set aside to use as a garnish. Set the remaining cilantro sprigs aside.
Step 3In a 3- to 4-quart pot, toast the coriander seeds and clove over medium heat until fragrant, 1 to 2 minutes. Add the ginger and bruised green onion sections. Stir until aromatic, about 30 seconds.
Step 4Remove the pot from heat for about 15 seconds to briefly cool, then pour in the broth. Add the water, cilantro sprigs, chicken, and salt. Bring to a boil over high heat, then lower the heat to gently simmer for 30 minutes.
Step 5After 5 to 10 minutes of simmering, the chicken should be firm and cooked through (press on it and it should slightly yield). Transfer the chicken to a bowl, flush with cold water to stop the cooking, then drain. Cover loosely so the chicken does not dry out.
Step 6When the broth is done simmering, strain it through a fine-mesh strainer into a 2-quart (2-liter) pot; line the strainer with muslin for superclear broth. Discard the solids. You should have about 4 cups broth. Season with fish sauce and sugar, if needed, to create a strong savory-sweet note.