Combine first 5 ingredients in a large saucepan; bring to a boil. Reduce heat, and simmer 20 minutes. Remove ginger and lemongrass; discard.
Meanwhile, heat oil in a large ovenproof skillet over medium-high heat. Add fillets, skin-side up; cook 2 to 3 minutes or until browned. Turn fillets over, and brush browned flesh of each fillet with 1 tablespoon hoisin. Place skillet in oven. Bake at 450º for 7 to 10 minutes or until desired degree of doneness.
Add carrots and zucchini to broth mixture. Simmer 5 minutes, and add cabbage. Simmer 2 more minutes or until cabbage is wilted. Add green onions and cilantro. Spoon broth into bowls; top with salmon. Garnish, if desired.