Peel the onion, cut it into very thin rings and place it in a preserving jar. Put the water, vinegar, sugar and salt in a small saucepan and heat over medium-high heat, stirring until the sugar has dissolved.

Pour the vinegar mixture over the onions until they are all covered. Seal the jar and let it sit at room temperature for 30 minutes, until the onions are soft and a vibrant pink.

Wash the cucumber, then lightly crush it with a rolling pin or the bottom of a pot to split the skin. Cut the cucumber into bite-sized pieces, place them in a colander, and mix with a little salt. Let the cucumber pieces sit for 10 minutes.

For the dressing, mix soy sauce, rice vinegar, oil, and sugar, and season to taste with chili and roasted garlic. Wash the spring onions and slice the green part into rings or strips (use the white part for something else).

Meanwhile, toast the sesame seeds in a dry pan over medium heat for a few minutes until golden brown. Remove the pan from the heat and immediately stir the hot sesame seeds into the dressing. Add the cucumber and spring onions, stir well, and serve topped with some red onions.