Heat a large pot over medium-low heat and add the olive oil. Stir in the onion, garlic, salt and pepper and cook, stirring occasionally, until the onion is translucent, about 5 minutes. Stir in the tomato paste, sugar , oregano and red pepper flakes, cooking for another 1 to 2 minutes while stirring. Add in the whole peeled tomatoes and stir. Break apart the tomatoes with a wooden spoon, then add the puree and stock, cover and cook over medium-low heat for 10 to 15 minutes.