Preheat oven to 350. Clean and poke with fork all over the potatoes, wrap each in foil and bake 1 hour, or done until tender. In a large skillet, heat oil, medium heat, add the onion, salt and pepper to taste, sauté two or three minutes on low heat, add garlic and cook about 30 seconds, add the sausage and break up meat into small pieces and cook until brown, 6 to 8 minutes. Add the marinara sauce and arugula. Stir in the marscarpone until mixture forms a creamy sauce. Remove the pan from heat and stir in Parmesan cheese. Season to your taste if necessary with salt and pepper. Cut the potato in half and/or slit the top, gently take out most of the potato and add a little butter gently smash a little, put back into the potato and spoon the sausage sauce into each potato. garnish with chopped parsley and serve.