Heat the olive oil in a medium pot or saucepan over medium heat. Add the onion, garlic, red pepper flakes, and salt and cook, stirring often, for 4 to 5 minutes, or until softened.

Add the tomatoes and their juices, crushing them against the sides and bottom of the pot so that the mixture is mostly smooth but still has some texture. Add the sugar and reduce the heat to low.

Simmer, stirring occasionally, for 20 minutes, or until thickened. Season to taste.