In a large pot heat the olive oil over medium heat. Add the onions and cook for 5 minutes or until translucent. Add the garlic and cook for another minute. Pour in the plum tomatoes and stir to combine. Add oregano, basil, sugar and salt and pepper and mix well. Bring to a simmer and cook for 25-30 minutes. Remove from heat and allow sauce to cool for 15 minutes.
Pass the sauce through a food mill or puree in batches in a food processor. Caution: proceed with care to avoid being burned by the hot sauce. Return pureed sauce to pot and heat to serve.