Prepare the aromatics and sauté - Slice the onion into 1-inch pieces. Grate the garlic and ginger using a microplane. Add the onions to a large pot, sprinkle with salt and pepper, and sauté in the oil until softened. Then add the garlic and ginger and sauté for minute or so.
Add the water and the squash - Add the water and squash to the pan and cook on medium-high heat until the squash is softened (if fresh) or thawed (if frozen).
Make the broth - Add the coconut milk and the green curry paste to the pan. Bring to a boil, then reduce and simmer until just beginning to thicken.
Poach the salmon - Cut the salmon into 2-inch pieces. Add the salmon pieces to the pot and simmer until cooked through, about 5 minutes or so.
Garnish and serve - Off the heat, add the lime juice and fish sauce. Ladle the curry into bowls and top with the chopped basil and cilantro. Serve with lime wedges.