Heat oil in large stock pot over medium heat. Add onion and sautee until softened, about 5 minutes. Add garlic and continue to sautee until fragrant, about thirty seconds.
Add all remaining ingredients, mix well and bring to a simmer. Lower heat and allow to simmer, uncovered for at least thirty minutes, adding a bit of water if mixture becomes too thick.
Ladle into bowls and top with scallions, cilantro or avocado slices.