Layer the vegetables in a large bowl, sprinkling each layer with salt, then cover and leave to stand overnight. The next day, lightly rinse and thoroughly dry them.
Mix the flour to a smooth paste with a little of the vinegar. Heat the remaining vinegar in a large saucepan with the sugar, spices and mustard powder over a low heat, stirring continuously, until the sugar has dissolved. Increase the heat and bring to the boil, then season the mixture generously with pepper and add the vegetables. Bring back to the boil, then reduce the heat and simmer, uncovered, for 10 minutes.
Remove the pan from the heat and gradually stir in the flour mixture. Return to the heat, bring to the boil and simmer for a further 5 minutes.
Ladle into warm, dry jars. Disperse any air pockets with a skewer or small knife and cover with screw-top lids. Label and leave to mature in a cool, dark place for at least 3 weeks.
To serve, partner with slivers of Parma ham.