Create the Teriyaki Sauce: In a small saucepan, combine the soy sauce, mirin, sake, brown sugar, minced garlic, and grated ginger. Whisk together over a medium heat until the sugar has completely dissolved.

Simmer and Divide: Bring the sauce to a gentle simmer and let it bubble for 2-3 minutes. Don't let it boil vigorously. Pour about 80ml (a third) of the sauce into a separate bowl or jug and set it aside to use as a marinade.

Thicken the Glaze: Return the saucepan with the remaining sauce to the heat. If you want a thicker glaze, whisk in the cornflour slurry and let it simmer for another minute until it has a consistency that coats the back of a spoon. Remove from the heat and set aside. This will be your basting glaze.

Marinate the Chicken: Place the chicken thighs in a shallow dish or a reusable food bag. Pour over the 80ml of reserved (un-thickened) marinade, ensuring all the chicken is coated. Let it marinate at room temperature for at least 30 minutes, or in the fridge for up to 4 hours.

Preheat the Grill: Prepare your barbecue for direct, medium-high heat, or heat a griddle pan over a medium-high heat on the hob. Lightly oil the grates to prevent sticking.

Grill the Chicken: Remove the chicken from the marinade, letting any excess drip off. Discard the leftover marinade. Place the chicken on the hot grill. Cook for 6-8 minutes on the first side, until well-marked and opaque halfway through.

Flip and Baste: Flip the chicken thighs over. Now, start brushing the thickened teriyaki glaze over the cooked side. Cook for another 6-8 minutes. In the final 2 minutes of cooking, flip the chicken once more and brush glaze on the other side. I find that basting only at the end prevents the sugars from burning while still creating a beautiful, sticky coating.

Check for Doneness: The chicken is cooked when the internal temperature reaches 74°C on a meat thermometer. According to the Food Standards Agency, chicken should be steaming hot with no pink meat visible.

Rest and Serve: Transfer the cooked chicken to a chopping board and let it rest for 5 minutes. This allows the juices to settle, ensuring the meat is succulent. Slice against the grain, garnish with sliced spring onions and toasted sesame seeds, and serve immediately with any remaining glaze.