Preheat the oven to 350º.
Carefully wash the greens, remove their stems and coarsely chop the leaves.
In a Dutch oven over medium heat, melt the butter. Add the onion and sauté until soft, about 5 minutes. Add the greens, garlic, and a pinch each of salt and pepper. Cook covered until the greens are wilted, stirring occasionally, about 10 minutes.
Spread the wilted greens in a layer over the cut surface of the pork. Lay the Andouille sausage lengthwise down the center of the roast and trim to fit. Reassemble the roast by rolling the loin around the sausage and greens, tie in several places with kitchen string.
In a small bowl combine the mustard, cayenne, sage and pepper. Melt the bacon fat or butter and whisk into the mustard mixture. Brush the loin generously with the mixture and place it in a roasting pan. Roast until the interior of the pork loin reads 155º-160º on a thermometer, about 1½ hours. Let rest for 10 minutes before slicing.