Preheat the oven to 400 degrees F. Season the potatoes with olive oil, salt and pepper. Place on a baking sheet and bake until tender, about 1 hour. Remove from the oven and cool. In a saute pan, over medium heat, add 2 tablespoons of olive oil. When the oil is hot, add the onions. Season with salt and pepper. Cook until caramelized, about 8 minutes. Remove from the heat and cool. Meanwhile, in a saute pan, over medium heat, add the 1 tablespoon of oil. When the oil is hot, brown the sausage, about 6 minutes. Using a slotted spoon, remove the sausage and drain on paper towels. Set aside. Using a sharp knife, remove the top 1/4 of each potato. Using a spoon, remove the flesh from each potato, leaving about 1/4-inch border on each potato. Set the hollowed potato shells, aside. In a mixing bowl, combine the potato flesh, butter, and cream fraiche. Season with salt and pepper. Using a hand masher, mash until smooth. Add half of the cheese, sausage, chives and parsley. Season with salt and pepper. Mix well. Fill each hollow potato with the potato/sausage mixture. Top each potato with the remaining cheese. Place on a baking sheet and bake until the potatoes are warm and the cheese melts. Remove from the oven and serve.