Heat olive oil in a large sauce pan set over medium-low heat. Add the onion and jalapeno pepper, and sweat (cook without browning) until the onion is soft and translucent, 7 to 10 minutes. Add the garlic and cook for additional 2 minutes. Stir in the mint. Let cool slightly.
Place the avocados in a food processor or heavy-duty blender, along with 1½ cups vegetable broth. Pulse until finely chopped.
Add the remaining broth, along with the onion mixture, salt and pepper. Blend until smooth.
Pour the soup into a medium-sized bowl and stir in the lite coconut milk.
Cover the soup and refrigerate for at least 1 hour. Serve chilled, topped with diced tomatoes (if desired).