Preheat oven to 400°F. Rub pork with fennel seed and salt. In an ovenproof skillet over medium-high heat, warm olive oil and brown pork. Remove to a plate. Add orange sections, onion, and fennel bulbs to the skillet and cook for about 5 minutes, stirring until softened. Place the pork on top and put the skillet in the oven for 15 minutes or until the internal temperature of the pork is 145°F. Remove from oven and transfer pork to a cutting board. Add butter, wine, and juice of 1 orange to the skillet and cook over medium heat for 2 to 3 minutes, or until slightly thickened. Cut pork into 8 slices and serve with oranges, vegetables, and pan juices.