Place the butter into a pre-heated oven steam setting for 1-2 minutes to soften.
Pick and chop the thyme sprigs, once the butter has softened but not melted remove the butter from the oven and add the chopped thyme and English mustard. Whisk everything together and set aside.
Cut two sheets of baking paper slightly larger than the fish pieces and place one sheet in the base of a solid baking tray. Place the fish on top of the baking paper and season with salt.
Smear the mustard butter over the rockling and cover the fillet with the other sheet of paper.
Place the tray into a pre-heated oven steam setting 75 °C for 14 minutes.
Wash the spinach leaves and place them on a perforated tray and set aside.
Once the timer goes off adjust the temperature to 100 °C steam setting and remove the fish. Place the spinach in the oven and
cook for 2 minutes.
Pour the juices collected in the bottom of the tray from the cooked fish into a small pot and place the pot on the stovetop and reduce by half.
Once the spinach has cooked remove from the oven and place the spinach into a serving bowl, place the fish on top of the spinach and pour over the reduce butter and serve.