In a large pot or dutch oven, saute the onions over medium heat until translucent. Add the garlic and stir for 30-60 seconds.
Add the remaining ingredients and simmer covered, for 20-30 minutes, or up to 2 hours, stirring occasionally.
Use an immersion blender to blend the sauce together. If you prefer a chunky marinara sauce, you can just use a wooden spoon to break the larger tomatoes into pieces instead of using a blender.