Place a rack in upper third of oven; preheat to 425°. Whisk 1 Tbsp. soy sauce, ½ cup mayonnaise, and 3 Tbsp. white miso in a large bowl to combine. Add 1½ lb. skinless, boneless chicken thighs (about 6), patted dry, and turn to coat.

Place 2 small leeks or 1 large leek, white and pale green parts only, sliced ¼" thick, and 1 lb. brussels sprouts, trimmed, quartered lengthwise, on a rimmed baking sheet. Drizzle with extra-virgin olive oil and season generously with kosher salt; toss to coat. Arrange chicken on top and roast until vegetables are slightly tender, 13–16 minutes.

Heat broiler (leave baking sheet in oven). Broil until vegetables are tender and charred in spots and chicken is cooked through and well-browned in spots, 9–12 minutes.

Meanwhile, whisk 1 Tbsp. unseasoned rice vinegar, remaining ¼ cup mayonnaise, and remaining 1 Tbsp. white miso in a small bowl. Set miso-mayo aside.

To serve, transfer chicken to a cutting board and coarsely chop. Drizzle remaining 2 Tbsp. unseasoned rice vinegar over brussels sprouts and leeks on baking sheet and toss to coat. Divide steamed jasmine rice evenly among bowls and top with brussels sprouts and leeks, then chopped chicken. Drizzle reserved miso-mayo over and sprinkle with toasted sesame seeds.