If using fresh onion/garlic: Heat olive oil in a large saucepan/braiser or Dutch oven over medium high heat. Add the onions and sauté until softened, 5-7 minutes. Add garlic and sauté for one additional minute. Combine the rest of the sauce ingredients in the pan. Bring to a vigorous simmer then reduce to a gentle simmer for 15 minutes, stirring often. If adding the tomatoes whole, use a potato masher to crush them occassionally until they reach desired consistency. Add chicken broth as needed to thin to desired consistency.
Taste and season to taste with sugar for sweeter, balsamic for tangier, red pepper flakes for spicier and/or salt and pepper.
Optional for a smooth sauce: Use an immersion blender or transfer sauce to the blender in batches and puree, taking care to leave a corner of the lid open and covered with a paper towel so the steam can escape or else it will explode.