Seared Salmon with Winter Vegetables and Kombu Broth

Heat 1 tablespoon oil in a large skillet over medium-high heat. Add onion, cut sides down, and cook, undisturbed, until dark brown, about 5 minutes. Transfer onion to a large Dutch oven or other large pot and add turnips, shallots, ginger, kombu, 1 tablespoon salt, and 8 cups water. Bring to a gentle simmer (do not boil or broth will be bitter) and cook until broth is light golden and onion is soft, 40 –50 minutes. Add vermouth to broth and simmer 5 minutes. Remove onion and kombu; discard. Add squash and simmer until just softened, 12 –15 minutes. Add turnip greens and simmer, stirring occasionally, until tender, about 2 minutes. Meanwhile, season salmon with salt and pepper. Heat remaining 1 tablespoon oil in a large skillet over medium-high heat. Cook salmon, skin side down, until skin is very crisp, about 2 minutes. Turn salmon and cook 30 seconds longer (salmon will be rare; it will continue to cook in the hot broth). Spoon broth and vegetables into shallow bowls and top with salmon, skin side up; squeeze lemon over. DO AHEAD: Broth and vegetables (without turnip greens) can be made 1 day ahead. Reheat and add greens just before serving.