Scrape the skin and blisters off the bitter melons with a vegetable peeler, just enough to remove raised parts. Cut the flesh into thin slices. Put the slices in a colander and sprinkle with sea salt. Leave to stand for 30 minutes, then rinse under cold running water to remove the bitter juices. Drain on kitchen paper and transfer to a plate. Sprinkle with the turmeric and toss to mix well.
Heat the oil in a large frying pan over a medium heat. Add the onion and stir-fry for 4–5 minutes. Add the curry and chilli powders, agave syrup and tomatoes, and continue to stir and cook for 8–10 minutes.
Stir in the bitter melon slices and cook, stirring, for 10–15 minutes. Season to taste and stir in the soy sauce. Serve immediately with rice.