Slice the cucumber into thin rounds and place in a bowl. Add 1 large pinch each of sea salt and sugar, 2 tablespoons of the rice vinegar, and massage it into the cucumber.
Place a dish on top inside the bowl and weigh it down with something heavy. Set aside to allow the excess water to drain out.
Rinse the rice well, until the water runs almost clear. Drain and place in a small saucepan along with 375ml of water.
Pop a glass lid on, bring to the boil, then cook over a low heat for 25 minutes, or until you can see holes in the surface of the rice (don’t be tempted to lift the lid). Take off the heat and leave, covered, for 20 minutes.
Fold 4 tablespoons of rice vinegar, the tablespoons of sugar and 1 pinch of salt into the rice.
Set the grill to high. Quarter the aubergine lengthways and score the flesh. Place flesh-side up on a foil-lined baking tray and grill for 15 minutes, or until almost tender.
Mix the miso and mirin with the remaining tablespoon of rice vinegar and 1 tablespoon of water. Brush half the glaze over the aubergine; grill for a further 8 to 10 minutes, brushing with the remaining glaze halfway, until tender.
Meanwhile, toast the sesame seeds (if using) in a dry frying pan until golden.
Place the aubergine on a bed of sticky rice. Trim and finely slice the spring onions, scatter over the aubergine with the sesame seeds (if using) and serve with cucumber pickle on the side.