Preheat a broiler with a rack set 4 inches from the heating element. Line a baking sheet with foil.
Whisk together 2 tablespoons of the soy sauce, the sugar and lemon juice in a medium bowl. Add the salmon and coat on all sides. Marinate at room temperature for 10 minutes, turning the salmon periodically. Place the salmon skin-side down on the prepared baking sheet. Broil until just cooked through, 4 to 8 minutes, depending on the thickness of the fillet. Cool enough to handle, then flake off bite-size pieces of salmon with a fork.
Combine 6 cups water, the remaining 1 tablespoon soy sauce, the ginger and scallion whites in a large saucepan. Bring to a boil and reduce to a simmer. Simmer for 10 minutes. Remove the ginger and scallions with a slotted spoon and discard. Whisk in the miso. Add the mushrooms and bok choy, and cook until the mushrooms are tender and the bok choy leaves wilt, about 2 minutes.
Mound 1 cup of the hot rice into four bowls. Ladle the vegetables and miso broth around the rice. Top each with some flaked salmon, sliced scallion greens, seaweed and a generous sprinkling of sesame seeds. Serve with sambal on the side if using.
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