Add the diced tomatoes, puree, oregano, chili flakes, and sugar. Mix well and allow the sauce to come to a boil. Lower heat until the sauce is just simmering. Allow it to simmer for a half hour. If you don’t have a half hour (I didn’t), let it simmer for at least five minutes. Salt to taste. (Optional: Puree sauce in a blender. I like a smooth tomato sauce, but if you like a little texture, you can skip this step! Whatevs.)