1. To make the pickles, slice the mushrooms thinly and cut the other vegetables into matchsticks. Toss them with the salt and sugar and leave on a sieve set over a bowl to draw moisture, for about 15 minutes.
2. Mix the soy sauce, mirin, rice vinegar, chilli and red pepper flakes in a cup or a small pan and heat to almost boiling, on the hob or in the microwave. Squeeze moisture from the vegetables, place them in a clean bowl and pour over the hot liquid. Stir and chill for at least 1 hour.
3. Cook the rice: rinse it with cold water several times, swirling the grains, till the water runs almost clear. Drain, place in a saucepan with a fitting lid and add the 280ml water. Bring to the boil stirring once or twice, put the lid on and turn the heat down to medium-low; you should see steam escaping from under the lid. Cook for 15 minutes without lifting the lid. Take the pan off the heat, keep it covered and leave it for 10 minutes or until the rice is not too hot to handle.
4. Meanwhile drain a couple of tablespoons of pickles (the rest can be used just as pickles), squeeze out moisture and chop them finely; you need only a small spoonful for each onigiri.
5. Prepare the glaze: mix all the ingredients except sesame seeds together in a small bowl.
6. Unless you have the onigiri shaping mould, use a plain round pastry cutter (mine was 6cm) and find a glass or tumbler whose bottom fits into it. Prepare a bowl of water for your hands and the tumbler – the rice is very sticky. Brush a baking tray with oil.
7. Place the pastry ring on a flat wet surface, pile a tablespoon of rice into it and press it down with the tumbler. Place a mound of chopped pickles in the middle and top with another tablespoon of rice. Dip the tumbler in the water and press its bottom down on the rice to pack in as densely as you can. Lift the ring and use the tumbler to push onigiri out onto the baking tray. Continue with the rest of the rice, wetting the tumbler, the ring and your hands often.
8. Preheat the grill to medium. Brush the tops of the onigiri with the miso glaze and place the tray under the grill for about 7-10 minutes until browned and crusty. Remove the tray from the grill, flip the onigiri over with a palette knife and brush the other sides. Sprinkle with sesame seeds; white, black or a mix. Return onigiri under the grill and cook for 3-5 minutes until the other side is crisp and deep golden.
9. Remove from the oven, cool down a little and serve with the remaining glaze as dipping sauce – or make some sweet and salty dip by mixing a spoonful of apricot jam with a spoonful of soy sauce.