Trim any excess skin or fat from the chicken thigh, then toss it with the sake. Cover and marinate for 30 minutes.

Dredge the marinated chicken thigh in flour, shaking off any excess.

Transfer the seared chicken straight to the freezer (don't leave it sitting out) for about 2 hours, until firm.

While the chicken chills, blanch the broccoli florets in boiling salted water for 90 seconds, then plunge them straight into an ice bath until fully cold. Drain well and set aside.

In a saucepan, combine the mirin, sake, shoyu, dark shoyu, sugar, smashed garlic, and sliced onion. Keep the heat on low, stir until the sugar dissolves, and simmer gently for 10 to 12 minutes until slightly sticky and coating the back of a spoon.

Turn off the heat and discard the garlic and onion. Pour half of the glaze into a small bowl, you'll glaze the chicken in two batches so the pan doesn't get crowded.

Remove the chicken from the freezer and lay it skin-side down on a chopping board. Slice through the meat on the diagonal into even strips using a gentle back-and-forth motion; the chilled meat holds its shape cleanly.

Lay half the sliced chicken into one bowl of warm glaze over medium-low heat. Cook for 2–3 minutes to let it soak up the sauce, then flip and cook the other side for 2–3 minutes, a little longer if the slices are still icy from the freezer, until glossy and fully cooked through (at least 74°C / 165°F). Remove from the heat right away so the sauce doesn't reduce too far or burn. Repeat with the remaining chicken and glaze.

Spoon warm rice into bowls, lay the glazed chicken on top, and tuck in the broccoli florets. Spoon any extra glossy sauce over the top and shower generously with toasted sesame seeds. Serve immediately.