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 tumeric. Bring to a boil and then add the pumpkin. Cook on a fast simmer until the pumpkin is tender, about 15 minutes, although different sorts of pumpkins can vary enormously in the time they take to cook; some squash take as little as 5 minutes.
As I mentioned, you can cook the curry up till this part in advance, maybe leaving the pumpkin with still a tiny bit of bite to it (it will soften and cook as the pan cools). Either way, when you're about 5 minutes away from wanting to eat, get ready to cook the seafood.
So, to the robustly simmering pan, add the salmon and prawns (if you're using the prawns from frozen they'll need to go in before the salmon). When the salmon and prawns have cooked through, which shouldn't take more than 3-4 minutes, stir in the green veg you're using - sliced, chopped or shredded as suits - and tamp down with a wooden spoon. When the pak choi is wilted, squeeze in the juice of half a lime, stir and taste and then 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; the point is the coriander goes in just before serving. Serve with more chopped coriander, for people to add to their own bowls as they eat, and some plain Thai or basmati rice.