In a large sauté pan, heat 2 tablespoons oil over medium. Add onion, garlic, ginger, ½ teaspoon pepper and ¼ teaspoon salt. Cook, stirring occasionally, until starting to soften, about 5 minutes. Add curry paste and cook, stirring frequently, 3 minutes.
Add coconut milk and 1 ½ cups water; bring to a boil over high heat, then reduce heat to medium-high and simmer until liquid is slightly reduced, about 5 minutes.
Lightly season salmon with ¼ teaspoon each salt and pepper. Stir in snow peas, then nestle salmon in sauce; reduce the heat to medium-low and simmer gently, covered, until just cooked through, 3–5 minutes (*see notes below).
Off heat, stir in spinach, fish sauce, 1 tablespoon lime juice and 1 teaspoon lime zest. Sprinkle basil and cilantro over top.
Serve salmon curry over rice and drizzle with toasted sesame oil, if desired. Serve with lime wedges and additional herbs.