Spinach sweet potato samosas

Name: Spinach & sweet potato samosas
Ingredients: 2 large sweet potatoes (about 500g), peeled and cut into small pieces; 1 tbsp vegetable oil, plus extra for brushing; 2 red onions, 1 chopped, 1 halved and finely sliced; thumb-sized piece ginger, peeled and finely chopped; 2 garlic cloves, crushed; 1 fat red chilli, finely chopped (optional); small bunch coriander, stalks finely chopped, leaves picked; 2 tbsp curry paste (we used balti); 2 tsp black onion (nigella) seeds; 200g bag spinach (or frozen spinach); 270g pack filo pastry (6 sheets); ½ cucumber; 150ml pot natural yogurt; mango chutney, to serve