How to Make It
In a large bowl, cover the marrow bones with water, add 3 tablespoons of kosher salt and refrigerate for 2 days, changing the salted water several times.
Preheat the oven to 425°. Bring a large pot of salted water to a boil. Cover 1 end of each marrow bone with a carrot slice; secure the carrot with kitchen string. Add the marrow bones to the pot and simmer very gently over low heat for 10 minutes. Remove the pot from the heat and let stand for 5 minutes. Transfer the marrow bones, carrot side down, to a large roasting pan.
Rub the turbot with the olive oil and season with salt. Set the fish on the bones, dark side up, and dot with 2 tablespoons of the butter. Roast the fish for about 50 minutes, or until the flesh is opaque throughout. Remove from the oven, cover loosely with foil and let stand for 10 minutes.
In a small saucepan, heat the Fragrant Chicken Jus. Gradually whisk in the remaining 2 tablespoons of butter and keep warm over low heat.
Using 2 large serving forks, remove the top skin from the fish, then loosen the fillet on each side of the backbone. Cut the fish into servings, set on warmed plates and spoon some of the sauce on top. Untie the marrow bones and set 2 on each plate. Serve immediately.