Heat oil in a large, deep skillet over medium heat. Add onion, and cook, stirring frequently, until softened, about 6 minutes. Stir in curry paste, and cook 30 seconds. Stir in coconut milk, sugar and salt, stirring until curry is dissolved, and bring to a simmer. Add fish, cover pan and simmer for 3 minutes. Carefully stir in spinach and fish sauce. Simmer until spinach is just wilted and fish is cooked through, 3 to 5 minutes more. Serve over Cilantro-Lime Rice. Cilantro-Lime Rice: Preheat oven to 350° F. In a medium-sized pot, combine 2 cups water and lime zest, and bring to a boil. Stir in rice. Cover with lid, and place in oven. Cook for 15 minutes. In a small microwave-safe bowl, place lime juice, butter and salt. Cook for 30 seconds or until butter has melted. Set aside while rice continues to cook in oven. Remove rice from oven, remove lid and place in a large bowl. Stir butter and lime juice mixture into rice; then stir in cilantro.