How to Make It
Preheat the oven to 350°. Roast the sweet potatoes for about 30 minutes, or until slightly tender. Let cool, then peel and slice the potatoes crosswise 1/3 inch thick.
In a large enameled cast-iron casserole, melt 2 tablespoons of the butter over moderate heat. Add the apples and onion and stir to coat. Cover and bake for about 15 minutes, stirring a few times, until tender. Transfer to a plate.
Add 1/3 cup of water and 2 tablespoons of the butter to the casserole and bring to a boil over moderately high heat. Add the greens, cover and cook until tender, about 5 minutes. Remove from the heat. Add the sweet potatoes, apples and onion and season with salt and pepper.
Raise the oven temperature to 425°. In a large, heavy-bottomed ovenproof skillet, heat the oil until shimmering. Season the pork generously with salt and pepper and add to the skillet, fat side down. Cook over high heat until golden brown on 3 sides. Turn the pork so the uncooked side is down and roast in the oven for 15 minutes, or until an instant-read thermometer inserted in the center registers 145°. Transfer to a platter, cover with foil and let stand for 10 minutes.
Melt 1 tablespoon of the butter in the skillet. Add the shallots and cook over low heat, stirring, until translucent, about 4 minutes. Raise the heat to moderate, add the wine and boil until almost completely evaporated. Add the chicken stock and boil until reduced by half. Whisk in the mustard, parsley and thyme. Remove from the heat and whisk in the remaining 2 tablespoons of butter. Season with salt and pepper.
Gently rewarm the sweet potato mixture. Slice the pork loin 1/2 inch thick. Spoon the sweet potatoes, onion and apples onto 4 plates and arrange the meat on top. Spoon the mustard sauce over the pork and serve.