We fill a wide saucepan with enough water to comfortably cover the vegetables and bring it to a gentle boil.
We add fennel, carrots, shiitake mushrooms, shimeji mushrooms, garlic and ginger.
We season lightly with salt and pepper and simmer for 10–15 minutes, until the vegetables soften and release their aromas.
We gently lower the croaker fillets into the pot, making sure they are submerged.
We reduce the heat to medium-low and poach for about 10 minutes, until the fish is just cooked and tender.
We carefully remove the fish and vegetables with a ladle.
We finish by adding lemon juice and lemon zest to the cooking liquid.
We serve the fish with the vegetables, drizzle with olive oil and garnish with spring onions and fresh dill.