In a large skillet, sauté the onions in olive oil over medium high heat until translucent. Add minced garlic and salt and pepper and cook for 1 minute, stirring often. Add the turkey sausage and cook until the meat has cooked through. Add in the tomato sauce and arugula. Let the arugula cook into the sauce until it reduces in size. Add in the mascarpone cheese and stir till combined. Remove the skillet from the heat, and stir in the parmesan cheese. Add pasta into the sauce, and finish with more parmesan cheese.