Skim the thick creamy top off the tin of coconut milk and put it, over medium heat, into a large saucepan or casserole with the curry paste.
Let it sizzle and, using a fork, whisk or wooden spoon, beat milk and paste together until combined.
Still beating gently, add the rest of the coconut milk, fish stock, fish sauce, sugar, lemongrass, lime leaves and turmeric.
Bring to a boil and then add the pumpkin.
Cook on fast simmer until the pumpkin is tender, about 15 minutes.
Add the fish fillet; when it's cooked through, about 3-4 minutes, stir in any green veg you're using - sliced, chopped or shredded as suits - and tamp down with a wooden spoon.
Squeeze in the juice of half a lime, stir and taste and add the juice of the remaining half if you feel it needs it.
Take the pan off the heat or decant the curry into a large bowl, and sprinkle over the coriander; if using.