Wash the squash, cut it in half and remove the seeds. Cut the flesh into 5 cm cubes and, with the skin downwards, put them into a saucepan. Pour over most of the dashi, add the mirin and sugar.
Bring to the boil, then gently simmer for 8 minutes under cover, turning over the squash pieces halfway through.
Add the soy sauce and the remaining dashi, then cook further until soft (under cover). This should take about 5 minutes.
Remove the squash pieces from the cooking broth, leave them to cool slightly.
Serve warm or at room temperature.