Place dried porcini mushrooms in a small bowl with enough warm water to cover. Soak 20 minutes or until rehydrated and chop.
Melt half butter in a medium frypan over medium heat. Place fish in the frypan and season with salt and pepper. Sprinkle with 1/2 the lemon zest. Pour half the lime juice over the fish and continue cooking 5 minutes.
Flip the fish and season again with salt and pepper. Sprinkle with remaining lemon zest and cover with remaining lime juice. Stir remaining butter, spring onions and porcini mushrooms into the frypan. Continue cooking 5 minutes, or until fish is easily flaked with a fork.