In a large pot, heat 2 Tbs. oil over medium heat. Add onion, ginger and garlic and season with salt and pepper. Cook, stirring occasionally until softened, about 3 minutes. Add miso and cook, stirring frequently, until miso is lightly caramelized, about 2 minutes.
Add coconut milk and 1 cup water or stock and bring to a boil over high heat (Note: if you prefer a thinner broth, add more liquid). Cook until liquid is slightly reduced, about 5 minutes. Taste and season with salt and pepper (yes, again). Nestle in fish, reduce heat to medium-low and simmer gently until just cooked through, about 5 minutes (Note: if using a heartier vegetable like bok choy, add it at this stage. For tender greens such as spinach, add in during following step). Turn off heat, stir in lime juice, and adjust seasoning if needed.
Divide rice and salmon curry among bowls. Top with more lime juice and herbs plus your toppings of choice.