Peel and coarsely chop tomatoes. Do this by hand and not food processor. Transfer tomatoes to a slow cooker.
Melt butter over medium heat in a medium skillet. Add the onion and garlic, and saute until softened but not browned. Scrape into slow cooker.
Cover and cook on low for 2 to 3 hours.
At the end of the cooking time, stir in herbs. Cover and cook an additional 10 minutes. Taste for salt and add the sugar, if desired. Serve on top of your favorite pasta, you will have enough sauce for 1 lb of pasta.
I also add olives to the sauce, during the last phase of cooking. Whatever is left over, I freeze.
Another great tip - when peeling the tomatoes use a vegetable peeler (one suited for softer skin) and it will make life so much easier!