Vibrant and refreshing, this is the soup you'll crave on summer's hottest days. Adding a little bread soaked in water, a traditional Spanish trick, thickens the soup without muting the flavors or making the final dish too heavy.
Tear bread into pieces; place in a bowl with 1 1/2 cups water; soak 5 minutes. Place bread mixture, shallots, oil, vinegar, salt, cucumbers, and tomatoes in a blender; blend until smooth. Combine crème fraîche, chives, and dill in a bowl, stirring with a whisk. Divide soup among 6 bowls; top evenly with crème fraîche mixture.