For the Soup
Combine all ingredients in a nonreactive container; cover and refrigerate overnight. Purée in a blender; strain through a medium mesh strainer. Return to container. Refrigerate until chilled. Adjust seasoning if necessary.
For the Garnish
Marinate the tomatoes, avocado and onion in lime juice and 1 tablespoon olive oil. Refrigerate until ready to serve.
Place a 2-inch ring in the center of a chilled bowl. Fill with 2 tablespoons marinated vegetables. Ladle cold soup around the ring; gently remove ring and serve.