Put the bread in a bowl and add water to cover. Let soak for 5 minutes until softened.
Meanwhile, place the almonds and garlic in a blender container (or, if using a hand-held blender, into a mixing bowl) and pulse until almonds are finely ground.
Squeeze out the water from the bread. Add to the food processor. Blend to a smooth paste.
With the motor running, add the olive oil in a slow stream, then the vinegar and salt.
Thin the ingredients with 1/2 cup of water to the desired consistency.
Place the blended contents in a tureen, wooden bowl or pitcher and add about 2 cups of water. Taste for seasoning, add more salt and/or vinegar if necessary. The soup should be fairly tangy.
Chill until serving time. Stir before serving into bowls. Garnish with grapes.
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