Fresh Tomato Sauce

In a large pot, melt butter over medium heat. Add onion and cook, stirring occasionally, until softened, 5 minutes. Add garlic and cook, stirring, until fragrant, 1 minute.  Add tomatoes and sugar and season with salt and red pepper flakes, if using. Bring to a simmer and cook, stirring often, breaking tomatoes up with a wooden spoon, until tomatoes are broken down and sauce thickens slightly, about 25 minutes. Add lemon juice.  Taste and season with more salt as needed.