Cut broccoli into small florets and halve larger pieces. Place in a microwave-safe container with 1 tbsp water and a pinch of salt. Cover loosely with plastic wrap and microwave at 600W for 2 minutes. Drain water. Slice onion into 1cm wide strips.

Cut chicken into bite-sized pieces and season with salt and pepper. Mix all spicy miso sauce ingredients in a bowl.

Heat sesame oil in a skillet over medium heat. Place chicken skin-side down and cook until browned. Flip chicken, add onion, and sauté briefly. Add sake, cover, and steam for 2 minutes.