Combine the tomatoes, melons, and about 1¼ cucumbers in a blender or food processor. (Reserve ¼ cucumber for garnish.) Process until smooth. Transfer to a bowl.
Stir in the orange zest and the ¼ cup mint. Whisk in the sour cream or crème fraîche and chill.
To serve, ladle into bowls and garnish with a bit of the chopped cucumber and a sprig of mint.</