For two portions:
Very, very finely chop the lemongrass - you want it to be almost like sand. Repeat with the ginger and the garlic, chopping until the pieces are almost a paste, and then the onion. The idea is that it should blend almost unnoticeably with the coconut milk later. If you have one, by all means puree the lot in a food processor or blender with the oil.
Heat the oil in a large saucepan. Add the chopped or pureed lemongrass, ginger, garlic and onion, the chilli flakes, paprika and cumin. Fry very gently, being careful that it doesn't start to brown or burn. Chop the chicken into bite-sized pieces and add them to pan. Give it all a good stir to get the chicken coated in the flavourings. Fry for about 5 mins, by which time the chicken should be almost cooked through. Add the coconut milk and nam pla. Let it simmer for another 5 minutes, then add the pepper and broccoli. Once the broccoli is cooked, squeeze in the lime juice, throw in the chopped coriander and check the seasoning. Add more nam pla if you think it needs more salt. Cook for the briefest minute to meld everything, then serve over noodles or rice, with some more coriander sprinkled over and a wedge of lime on each bowl if you are feeling fancy.