Season the salmon: Season the salmon lightly with salt and pepper on both sides. Refrigerate for about 20–30 minutes while you prepare the vegetables.
Start the vegetables: Heat the oil in a large, deep saucepan or Dutch oven over medium heat. Add the onion and potatoes. Cook for about 5–7 minutes, stirring occasionally.
Add mushrooms and aromatics: Add the mushrooms and bell pepper and cook for another 3–4 minutes, until the mushrooms begin to soften.
Add the garlic and ginger and cook for 1–2 minutes, stirring frequently.
Make the miso broth: Add the miso paste and stir for about 1 minute. Pour in the coconut milk, soy sauce, and mirin. Stir well to combine.
Simmer the potatoes: Bring the broth to a gentle simmer. Reduce the heat to low/medium-low, cover, and cook for approximately 15–20 minutes, or until the potatoes are almost fork-tender. Stir occasionally.
Add the salmon: Gently nestle the salmon fillets into the broth, making sure they are partially submerged. Cover and cook for approximately 6–10 minutes, depending on the thickness of the salmon. Avoid vigorous boiling so the salmon stays intact.
Check for doneness: The salmon should be opaque and flake easily when gently pressed. The potatoes should be completely tender.
Serve: Carefully transfer the salmon and vegetables to shallow bowls. Spoon plenty of the coconut-miso broth over everything.
Finish with fresh cilantro, a squeeze of lime, and a small drizzle of chilli oil if desired. Serve with rice if you want an extra-hearty meal.