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Heat oven to 200C/180C fan/gas 6. Put the potatoes in a saucepan of water, bring to the boil and cook for 8 mins. Drain and leave to steam dry.
Put the potatoes in one roasting tin and the radishes into another. Divide the oil between them, along with the mustard seeds and chilli flakes. Season and give everything a good mix. Put in the oven and roast for 25 mins, then take out the radishes and roast the potatoes for an extra 10 mins until crisp. Meanwhile, toast the pumpkin seeds in a dry frying pan.
Spread the yogurt onto a sharing platter. Pile on the potatoes and radishes, then scatter over the spring onions, pumpkin seeds and a few more chilli flakes, if you like.