Place the cauliflower, cucumber, zucchini and onion in a large glass bowl and sprinkle over the fine salt. Stir to mix, then cover and place in the fridge if the weather is humid.
The following day, rinse with very cold water and pat dry with kitchen paper.
Place the vinegar and sugar in a large saucepan and place over low heat. Add the mustard paste, cumin seeds, mustard seeds and turmeric. Cook very gently, stirring occasionally, until the sugar dissolves. Increase the heat and bring to the boil, then season with freshly ground black pepper. Add the vegetables, return to the boil, then reduce the heat and simmer for 10 minutes. Remove from the heat.
Place the cornflour in a small bowl and mix with a little of the hot vinegar liquid until you have a smooth paste. Pour into the vegetables and stir. Place vegetables back on medium-high heat and bring up to the boil, then reduce the heat to a simmer for 5 minutes to cook out the cornflour.
Place the piccalilli into hot, sterilised jars and seal. Keeps for several months.