Fill a large saucepan with water, salt well and bring to a boil. In a large bowl, prepare an ice bath.
Rinse the spinach with cold water to remove any dirt, especially around the roots. Remove any hairs from the roots. Cut the spinach into 7 cm (2 3/4 inch) long pieces. Keeping the purple roots intact. Split the roots in half horizontally and keep separately.
In a medium bowl, combine the dressing ingredients and stir until the sugar has dissolved. Set aside.
Blanch the spinach in two separate batches. In the boiling salt water, blanch the spinach stems with leaves until just wilted, about 1 minute, remove the spinach from the water and plunge in the ice bath. Repeat with the spinach stems with roots, but cook for 1-2 minutes before plunging into the ice bath. Remove from the ice bath, drain well and gently squeeze out any excess water.
Gently loosen the clumbs of spinach with your fingers and transfer to a bowl. Toss well with the dressing. Cover and chill for about an hour to allow the flavours to mellow before serving.