Heat oven to 200C/180C fan/gas 6. Toast the mustard seeds in a dry pan on the hob for a few mins until they begin to pop. Carefully (as it will spit) add the vinegar, sugar and seasoning, then stir to dissolve. Leave to cool, then drizzle the dressing over the vegetables and give everything a stir. Can be stored in an airtight container in the fridge for up to 7 days.
Toss the nuts with the rest of the ingredients, except the rosemary, and season generously. Spread out on a baking tray and roast for 15 mins, stirring once. Add the rosemary and roast for 5-10 mins more until the nuts are golden – keep an eye on them as they burn easily.
Allow to cool completely, stirring occasionally to stop them from sticking. Can be stored in an airtight container in the fridge for up to 7 days. Sprinkle over the slaw before serving.