How to Make It
In a medium skillet, heat 1 tablespoon of the oil. Add the endives, cut sides down, season with salt and cook over moderate heat, until browned on the bottom, about 3 minutes. Add 1 more tablespoon of oil to the skillet. Turn the endives and cook over low heat until tender and golden brown, about 4 minutes longer. Transfer to a plate and keep warm.
Add 1 tablespoon of oil to the skillet. Add the fennel, cut sides down, season with salt and cook over moderate heat, until browned, about 3 minutes. Add 1 more tablespoon of oil to the skillet. Turn the fennel and cook over low heat until tender, about 4 minutes longer. Transfer to the plate with the endives.
Grate 1 teaspoon of zest from the oranges. Peel the oranges with a sharp knife, removing all of the bitter white pith. Working over a bowl, cut in between the membranes to release the sections. Squeeze 1/4 cup of juice from the membranes into a small saucepan. Add the orange zest, vinegar, shallot and tarragon stems. Boil over high heat until reduced to 2 tablespoons, about 3 minutes. Remove from the heat and discard the tarragon stems. Whisk in the butter, 1 tablespoon at a time, until the sauce is creamy. Season with salt and pepper and stir in the chopped tarragon.
In a large nonstick skillet, heat the remaining 1 tablespoon of oil. Season the cod with salt and pepper and cook over moderately high heat until richly browned and just cooked through, about 4 minutes per side.
Transfer the cod, endives and fennel to plates. Arrange the orange sections around the cod. Reheat the sauce, if necessary, but do not boil. Pour the sauce over the cod, garnish with whole tarragon leaves and serve.