Mix the cauliflower, squash, carrots, pepper and shallots in a large colander, sprinkle over the salt and toss to coat. Put colander over a large bowl, cover and leave for 12hr, or overnight, to draw out excess moisture.
When the vegetables have been salted, in a large pan heat the vinegar, sugar, ginger, chilli, coriander and mustard seeds. Bring to a simmer, stirring to dissolve, then add vegetables and simmer for 2-3min, until partially cooked but still crunchy. Lift vegetables out using a slotted spoon and set aside (reserve liquid in pan).
In a small bowl, mix cornflower, mustard powder and turmeric with 100ml water to a smooth paste. Whisk into the pan and heat, stirring constantly, for a few min until bubbling and thickened.
Return vegetables to the pan; mix. Spoon into sterilised wide-neck jars, making sure each jar has an even distribution of vegetables and sauce. Seal and cool completely.
