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First, make the pickled onions. Pour the vinegar and 50ml water into a small saucepan. Add the sugar, 1 tsp salt and the mustard seeds and simmer over a medium heat, stirring occasionally until the sugar has dissolved. Put half the onion into a heatproof bowl and pour the hot liquid over. Leave to cool slightly, then cover and leave to pickle in the fridge for 2 hrs.
Bring a large pan of salted water to the boil and cook the potatoes for 10 mins. Add the cauliflower and beans, and cook for 2 mins more, then drain.
Heat a depth of 3cm sunflower oil in a small heavy-based saucepan until a small cube of bread dropped into the oil turns golden within 20 seconds. Carefully drop the rest of the onion into the oil and fry for 1-2 mins, or until golden and crisp. Transfer to a sheet of kitchen paper and sprinkle with salt – this will help keep the onions crisp.
Mix the piccalilli and olive oil together in a bowl. Toss with the cooked potatoes, cauliflower and beans, along with the radishes and cucumbers. Toss in the pickled onions, then tip into a serving bowl and top with the crispy onions.