In a large pot, place the olive oil and butter over medium heat. Add the onion and cook, stirring occasionally, until browned. Stir in the tomatoes and garlic. Season with sugar, salt, and pepper. Once bubbling, reduce heat to a simmer and cook, stirring occasionally, until the tomatoes break down into a sauce, 45 minutes to 1 hour. If desired, use a potato masher to create a smoother sauce. Season to taste and stir in the parsley.