Bring a large pot of water to a boil and cook the noodles according to the package directions. If you time it just right, your sauce will be done at the same time your noodles are finished cooking. To make the sauce, heat 2 tbsp. extra virgin olive oil in a large skillet over medium-high heat. Add the onions, 1 tsp. salt and 1/2 tsp. pepper. Cook, stirring frequently, until softened, about 3 minutes. Add the garlic and cook until aromatic, about 30 seconds. Add the sausage and break up the meat into 1/2" pieces. Cook until browned and cooked through, about 6 to 8 minutes. Add the marinara sauce and arugula. Bring the mixture to a boil and cook until the arugula has wilted, about 2 minutes. Add the mascarpone cheese and stir until the mixture forms a creamy sauce. Remove from the heat and stir in the Parmesan. Season with salt and pepper, if needed. Return the noodles to their cooking pot and add the sauce. Stir well until combined and the noodles are heated through. Serve immediately with the parsley sprinkled on top of each serving.