In a medium sauce pot, heat olive oil over medium heat.
Add onion and cook for about 5–6 minutes. (Make sure onion is translucent and does not brown.)
Add garlic and summer squash and sauté for about 8–10 minutes, until squash is soft.
Add the remaining ingredients (except for the chopped green onion) and bring to a boil.
Turn heat to low and let simmer for about 10 minutes.
Remove from heat.
Once cooled slightly, blend mixture (in food processor or with immersion blender) until smooth.
Serve warm or chilled, topped with chopped green onions.