Put the garlic, green pepper, tomatoes, bread, olive oil, vinegar, and salt in a food processor. Pulse until the ingredients begin to puree (if the bread is hard, it may bounce about and take a while to break down); continue processing until the mixture is as finea puree as possible, 3 to 5 minutes.

Pass the soup through a large fine sieve set over alarge bowl, pressing until only solids remain in the sieve; discard the solids. Stir in 1/4 to 1/2 cup water, or enough to give the soup the consistency of a thin milkshake. If you want a thicker soup, add less water, or none at all. Add more salt or vinegar to taste. Cover and refrigerate until well chilled (or serve it immediately with a few ice cubes in each bowl).

Ladle the gazpacho into chilled bowls or cups. Grind fresh pepper on top, if you want, and pass bowls of diced cucumber and onion, if using, so people can garnish their own.