Cook the potatoes in a pan of boiling water for 6 mins, or until tender. Drain and leave to steam-dry. Meanwhile, put the spinach in a heatproof bowl with 1 tbsp water. Cover and microwave for 3 mins on high, or until wilted.
Heat the rapeseed oil in a medium non-stick frying pan. Add the onion and cook over a medium heat for 10 mins until golden and sticky. Stir in the garlic, ground coriander, ground cumin, mustard seeds and turmeric, and cook for 1 min more. Add the tomatoes and wilted spinach and cook for another 3 mins, then add the potatoes.
Heat the grill to medium. Lightly beat the eggs with the chilli and most of the fresh coriander and pour over the potato mixture. Grill for 4-5 mins, or until golden and just set, with a very slight wobble in the middle.
Leave to cool, then slice into wedges. Mix the mango chutney, yogurt and remaining fresh coriander together. Serve with the frittata wedges.