Peel the ginger and garlic, chop the tomatoes, then place in a blender and blitz until smooth. Chop the potatoes into 5cm chunks. Pour 2 tablespoons of vegetable oil into a large non-stick frying pan over a medium–high heat. Add the potatoes and cook for 10 minutes, or until golden, gently squishing them with a potato masher to increase the surface area and give you more crispy bits. Trim and roughly chop the spring onions, and deseed and finely slice the chilli (how much you use depends on how hot you like it!). Pick the coriander leaves and reserve, then finely chop the stalks. Add the cumin and mustard seeds to the pan and cook for 1 minute, until the cumin starts to darken. Add the spring onion, chilli and coriander stalks and cook for 2 minutes, then add the blitzed ginger mixture, the rest of the spices and a pinch of seasoning. Sauté for 4 to 5 minutes, or until fragrant. Serve the crispy Bombay potatoes with dollops of yoghurt, and the coriander leaves scattered over.