Heat half the oil in a deep, lidded frying pan and fry the onion with a pinch of salt for 5 minutes until soft. Stir in the garlic, ginger, mustard seeds, turmeric, coriander and ground cumin, and fry for another 2-3 minutes until fragrant. Stir in the coconut milk, half a tin of water and the mashed banana. Bubble for 5 minutes until the sauce starts to thicken slightly.