Put the sweet potatoes in a large bowl, cover with cling film and microwave on High for 8 minutes or until soft. Meanwhile, heat the oil in a large pan, add the chopped onion and cook for a few minutes to soften. Stir in the ginger, garlic, chilli (if using) and coriander stalks, stirring for a couple minutes more until fragrant. Add the curry paste and half the black onion seeds to the pan, stir for 30 seconds or so until fragrant, then add the spinach and 2-3 tbsp water. Cook the spinach until wilted, then add the sweet potato and any liquid from the bowl. Season well and mash everything together with the back of a spoon, leaving some chunky bits of potato. Leave to cool completely. Unroll the pastry and pull out two sheets to work with – keep the rest covered with a tea towel to prevent it from drying out. Brush both sheets with a little oil and scatter some of the black onion seeds over one sheet. Put the other sheet on top. With the shortest side facing you, cut down the centre to make two long strips. Scoop a sixth of the sweet potato mixture onto the top right-hand corner of the filo in a rough triangle shape. Fold the pastry over on an angle, continuing down the length of the pastry until you reach the bottom and have a neat triangle encasing the filling. Trim off any excess pastry with a knife. Repeat to make six samosas. Heat oven to 200C/180C fan/gas 6. Put the samosas on a baking tray lined with baking parchment. Brush with a little more oil and sprinkle over the remaining black onion seeds. Bake for 25-30 minutes or until deep golden brown. Meanwhile, peel the cucumber into ribbons, then toss with the sliced onions and coriander leaves. To serve, dollop some yogurt onto each plate, top with two samosas, a mound of the cucumber salad and mango chutney.