Heat oil in a wok or deep-sided frying pan on a medium heat. Add the shallots or brown onion, garlic and ginger and cook for 2-3 minutes. Add the spice and cook for another minute or so, being careful not to burn it.
Add the fish sauce, sugar, coconut milk, stock, lemongrass and lime leaves (this is optional) and stir until all the ingredients are well combined.
Bring the heat to low. Add the sweet potato and aubergine, (and mushrooms, if using) stir well, cover with a lid and simmer for 10 minutes. If they are still too firm, cook for another few minutes.
When the aubergine and sweet potato are nearly done, add the broccolini and cook for 2-3 minutes. They should still have a bite to them.
Add the red peppers and turn the heat off. Check the seasoning and add a splash more fish sauce if needed and a good squeeze of lime. Cover with a lid and let it rest for 2-3 minutes.
Sprinkle a small handful of cashews on the dish and serve immediately with rice, chopped coriander and more lime if needed.
