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Heat oven to 200C/180C fan/gas 6. Heat a non-stick frying pan over a medium-high heat, add the onions, thyme leaves, mustard seeds and 1 tbsp water, and cook for 10-15 mins. If the onions start to catch, add a splash more water. Once the onions are soft, add the vinegar and bubble for 1 min.
Meanwhile, bring the stock to the boil in a medium pan. Add the celeriac and parsnips, and simmer for 6 mins, stirring occasionally. Layer the vegetables with the onions in a large ovenproof dish. Pour over enough stock to come halfway up the vegetables, then bake for 30 mins until golden and cooked through.