Heat your oven to 200°C/Fan 180°C/Gas 6. Place 1 tsp turmeric, 1 tsp black mustard seeds and a pinch of chilli powder (it's spicy, so use as much or as little as you prefer) in a bowl. Add a pinch of salt and pepper and stir together.
Scrub the potatoes and chop them into quarters, or eighths if large, and arrange on a large baking tray. Sprinkle over half the spice mix and ½ tbsp olive oil. Toss to mix. Roast for 15 mins.
While the potatoes roast, tip the cashew nuts into a bowl and cover with 200ml boiling water. Set aside for 15 mins to soften.
Break the broccoli into florets. Slice the stalk into small chunks, trimming off the dry end, and add everything to a bowl. Drizzle over ½ tbsp olive oil and the remaining turmeric spice mix. Toss to coat. Add the broccoli to the potatoes and return to the oven for a further 15 mins.
Pour the cashews and water into a food processor. Crumble in the coconut cream and blitz well till finely chopped. No processor? Crumble the coconut cream into a bowl and stir in 200ml boiling water to melt the coconut cream. Drain and finely chop the cashews and stir them into the coconut cream to make a thick sauce. Set aside.
Peel the onion and finely chop it. Halve the chillies and scoop out the seeds and white pith for less heat, then finely slice the chillies. Peel and grate the garlic and ginger.
Pour 1 tbsp oil into a large frying pan and set over a medium heat. Add the onion, garlic, ginger, chilli and a little salt and pepper and fry for 3-4 mins, stirring, till the onions have softened. Stir in 1 tsp each garam masala and fennel seeds, and cook for a further 1 min.
Add the cashew cream to the pan and combine well. Take the potatoes and broccoli out of the oven and add to the pan, making sure to include any stray loose spices from the bottom of the baking tray. Cook for 3-5 mins till everything is warmed through, stirring the vegetables to ensure everything is coated.
Tear up the fresh coriander and stir most of it into the curry. Pile the broccoli and potato curry into bowls and top with the remaining coriander.