Peel and cut each garlic clove in half. Remove the green shoot inside, if there is one. Fill a small saucepan with cold water and add the garlic. Bring the water to a boil. When the water begins to boil lift the garlic out of the water and into a bowl of ice water to quickly cool it. Repeat this twice, always starting with cold water in the saucepan.
To food processor bowl, add shallot (peeled and cut in half) along with cucumber (peeled if not using thin-skinned) cut into pieces and bell peppers seeded and cut into pieces. Cut tomatoes into large wedges and add to food processor. Add garlic. Tear bread into pieces, add to processor bowl and pour in water. Process everything together until finely chopped. Add oil, vinegar and mayonnaise and blend until creamy. Season with salt and pepper. Chill at least two hours. Serve in chilled bowls topped with a drizzle of olive oil and croutons.