Crush the garlic and salt to a pulp, using either a mortar and pestle or the side of a large knife. Reserve a few walnuts. Using a food processor (or a mortar and pestle, if you want), briefly pulse the rest of the walnuts and then the bread. Transfer to a bowl.
Add a couple tablespoons of olive oil to the paste and then all the yogurt, whisking well. Add the lemon juice. Grate and add the cucumbers.
Let cool in the fridge for an hour or more, if you have it, and then add enough very cold water to make it into a spoonable soup. Chop the reserved walnuts and use for a garnish, along with the dill or mint, some lemon wedges and the olive oil.