Heat the oil in a frying pan over medium heat. Add the onion and cook, stirring, for 5 minutes or until soft. Add the curry paste and peanut butter. Cook, stirring, for 1 minute. In 4 batches, add the coconut milk, stirring well after each addition until combined. Add the 2 lime leaves, sugar and fish sauce.
Place the beef in a slow cooker. Pour over the onion mixture and stir until combined. Cover and cook on Low for 7 hours. Add the eggplant and beans. Cover and cook for 1 hour or until the vegetables are tender.
Top the curry with extra lime leaves. Divide among serving bowls. Serve with roti and rice.
