Step 1

In a large saucepan, combine water and salt, stirring to dissolve salt. Add chicken, scallion, and ginger and set over medium-high heat until water temperature reaches between 150 and 160°F on an instant-read thermometer; adjust heat to maintain water temperature in the 150–160°F range. (It's okay if the temperature bounces around a little, but try to keep it above 150 and below 170°F.) Cook until thickest part of chicken registers 150°F on an instant-read thermometer, about 1 hour. Remove chicken from broth and let rest. Strain broth through a fine-mesh strainer and save for sipping or another use.

Step 2

Meanwhile, in a small bowl, whisk together oil, lemon juice, miso, soy sauce, and sesame oil until smooth. In a medium bowl, toss watercress with just enough miso dressing to lightly coat.

Step 3

Remove and discard chicken skin and cut chicken meat from bones; discard bones. Slice chicken crosswise into 1/2-inch pieces. Scoop rice into bowls and top with watercress and chicken slices. Drizzle miso vinaigrette onto chicken and garnish with sliced scallions. Serve.