Cut the chicken thighs into bite-sized pieces. In a bowl, coat them with 1 tablespoon of the olive oil, salt and pepper.
Thread the chicken onto the skewers, not packing them too tightly.
For the teriyaki sauce, heat soy sauce, brown sugar, mirin, and sake in a small saucepan over medium heat. Bring to the boil, then simmer on low for 3-4 minutes or until the sugar dissolves and the sauce thickens. Set aside.
Slice the cauliflower into 3/4" slices. Center slices will stay together better. generally you can get about 3 steaks from one head. Keep the florets for another use.
Put the remaining olive oil, paprika, salt, onion powder, and pepper in a bowl. Baste the cauliflower steaks and bake them in the oven on 425°F (220°C) for 25 minutes, flipping once during cooking. Baste with any remaining olive oil during cooking for an even crispier result.
Cook the skewers on a grill pan or outside on the BBQ. Heat a large non-stick frying pan over medium heat. Cook skewers for 2 minutes per side then start to baste them with the teriyaki sauce on both sides. Continue cooking and basting for 2 more minutes per side.
Transfer skewers to a serving plate with the cauliflower. Top with spring onion and sesame seeds. Drizzle on more teriyaki sauce.