Sprinkle fish with salt and sugar. Let stand for 10 minutes. This will stiffen the fish to prepare it for sous-vide cooking; otherwise, it will get mushy. (Skip this step if not using sous vide.)
Set up a water bath and place a sous vide precision cooker in it. Set the precision cooker to 134 degrees Fahrenheit (56 degrees Celsius).
Season fish with salt and pepper. Place fish and one tablespoon oil in a large zip-top or vacuum-sealed bag, keeping the fish in a single layer. Place the bag in the water bath and set the precision cooker timer for 30 minutes.
To finish the fish, heat a frying pan with light olive oil. Add fish, skin side first, and sear for about one minute. Reduce heat slightly so the skin doesn’t burn. Continue searing, watching the heat and turning it up or down as needed to keep the fish sizzling gently. When the skin is crisp, it will release easily from the pan, so don’t move the fillets too soon.
No sous vide? You can also make this dish on the stove: Heat a cast-iron skillet with two tablespoons butter. Allow the butter to foam up, then add fish, skin side down. Cook until skin is golden (about three minutes). Turn the fish over and cook about two more minutes.
In a medium pot over medium heat, add two tablespoons butter, leeks, and garlic. Cook about five minutes.
Add carrots, sweet potato, ginger, thyme, and bay leaf. Sauté another five minutes.
Add broth. Bring to a boil, then reduce to a simmer. Cook 40 minutes or until vegetables are tender.
Discard bay leaf and purée the sauce with an immersion blender. Stir in honey, heavy cream, nutmeg, salt, and pepper. Finish the sauce by adding the remaining two tablespoons butter.
To save time, start preparing the sauce during the sous-vide cooking. The string beans can also be prepared during that time.
In a large sauté pan over medium-high heat, add all ingredients. Sauté about five minutes or until beans are just tender (they should have a somewhat crispy texture).
To serve, ladle about one-quarter cup sauce over each plate and top with prepared sea bass and string beans.