Miso Mayo Chicken Bowl

Name: Miso-Mayo Chicken Bowl
Ingredients: 1 Tbsp. soy sauce; ¾ cup mayonnaise, divided; 4 Tbsp. white miso, divided; 1½ lb. skinless, boneless chicken thighs (about 6), patted dry; 2 small leeks or 1 large leek, white and pale green parts only, sliced ¼ thick; 1 lb. brussels sprouts, trimmed, quartered lengthwise; Extra-virgin olive oil (for drizzling); Kosher salt; 3 Tbsp. unseasoned rice vinegar, divided; Steamed jasmine rice and toasted sesame seeds (for serving)