Remove the fish from the fridge and leave on the side for 1hr before cooking.
Season with a generous amount of salt. Do not remove the head or skirt.
Preheat the oven to 180ºc with fan. Once preheated switch to grill setting.
Place fish on the middle rack dark side up and cook for 10 minutes.
Flip and cook for a further 10 minutes.
If you have a thermometer test the thickest part of the fish - if 55º or above the fish is cooked. If using a fork check that the fish flakes easily.
Once cooked turn the oven off and leave fish to rest inside for 5 minutes with the door closed.
Bring a large pot to boil. Add a pinch of salt to the water.
Cut the spring greens in half and blanch for 2 minutes.
Once blanched drain the pot and retain 300ml of cooking water, preserving the greens in the pot.
Add butter to pot and greens and toss.
Add the 300ml of reserved cooking water and stir to emulsify.
Season with salt and lemon juice to taste.
Plate the fish and greens together and serve immediately.