Heat oil in a large frying pan over med-high heat, add cumin and mustard seeds and cook for a few minutes until popping. Add onion and chilli. Cook for around 5 minutes, stirring often until translucent and lightly golden. While they are cooking, drain the eggplant, rinse under cold water in a sieve and pat dry with the clean towel or paper towels. Add turmeric and drained eggplant to the onion mixture and cook for 2 minutes before turning down to low, covering with a lid and cooking for a further 5-7 minutes until golden and tender. Stir a couple of times during cooking to prevent the mixture catching on the bottom.