Saute the diced onion in a little bit of olive oil, butter, or cooking spray, in a medium saucepan over medium heat. (If you like garlic, you could add a few diced garlic cloves too).
Add the stewed tomatoes and the tomato paste. Stir to combine.
Add the brown sugar. Start with just 1/2 a cup. If you like it sweet, keep adding more. Here is when you can add salt, pepper, and any herbs like basil and oregano.
Turn the heat down to low and simmer for at least an hour. Stir every so often. The stewed tomatoes will keep breaking down the longer you cook it, so you can keep going for a less chunky sauce. The perfect sauce for any pasta that can also be used as a pizza sauce, or for meatballs, etc.