Skim the thick creamy top off the can of coconut milk and put it into a large saucepan or casserole with the curry paste, over medium heat. Let it sizzle and, using a fork, whisk or wooden spoon, beat cream and paste together until combined. Still beating gently, add the rest of the coconut milk, fish stock, fish sauce, sugar, lemongrass, lime leaves (if using) and turmeric. 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 can take a little as 5 minutes.
As I mentioned, you can cook the curry up until this part in advance, maybe leaving the pumpkin with a tiny bit of bite to it (it will soften and cook as the pan cools). Either way, when you're about 5 minutes from wanting to eat, get ready to cook the seafood.
So, to the robustly simmering pan, add the salmon and shrimp (if you're using frozen shrimp they'll need to go in before the salmon). When the salmon and shrimp have cooked through, which shouldn't take more than 3 to 4 minutes, stir in any green vegetable you're using - sliced, chopped or shredded as suits - and tamp down with a wood spoon. When the bok choy is wilted, or other green vegetable is cooked, 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 and pour the curry into a large bowl, and sprinkle over the cilantro; the point is that the cilantro goes in just before serving. Serve with more chopped cilantro for people to add their own bowls as they eat, and some plain Thai or basmati rice.