Run the bread under cold running water to saturate. Squeeze dry and crumble into a bowl.
In a food processor, process the garlic with the salt to form a paste, scraping down the sides as needed. Add the walnuts and process until mealy. Add the soaked bread and 2 tablespoons of the vinegar and process. With the machine running, add the oil in a steady stream and process until creamy. (If the mixture is too thick, first adjust, to taste, with vinegar,then add water 1 to 2 teaspoons at a time.)
Adjust the seasoning, to taste, and transfer to a decorative bowl for serving with the grilled fish.
Yield: about 1 1/2 cups