Bring water to a boil in a large pot. Heavily season the boiling water with salt and add pasta, stirring occasionally. Cook according to package instructions.
While the water is boiling start the sauce. Add olive oil and diced onions to a large skillet over medium-high heat and cook until soft and translucent, about 5-7 minutes. Add garlic and cook for 30 seconds until fragrant.
Stir in tomato paste and cook for 1 minute. Add diced tomatoes, turn the heat to low and simmer for 10 minutes, stirring occasionally. Add a pinch of sugar to balance the acidity of the tomatoes.
Stir in butter cubes, then add the parmesan.
Reserve some pasta water, drain the pasta and add them to the sauce. Stir to combine.
Add pasta water to the sauce to thin it if necessary. Stir in arugula right before serving and toss to coat. Taste for seasoning.
Serve with shaved Parmesan cheese, freshly ground black pepper and pine nuts.