Mix the salt and sugar. Sprinkle over the fish.
Let the fish sit under plastic wrap at room temperature for about 1 hour.
Wipe off the salt and sugar mixture.
Fry it skin side down for about 10 minutes on medium heat. Flip it over and continue.
Remove the pan from heat , put on the lid (or cover the pan with tin foil) and let it sit for about 15 minutes.
While waiting, peel the carrots and parsnips if needed. Finely chop the red onion.
Put the carrots, parsnips, rosemary and olive oil in a saucepan and cover it with water.
Season generously with salt and pepper. Add a tablespoon of butter and put the saucepan on the stove.
Boil until the carrots are soft without the lid on. Stir in another tablespoon of butter.
Season with salt and pepper and mix in the chopped herbs.