How to Make It
Working with one cucumber half at a time, use the side of a large knife to lightly crush the cucumber. Repeat with remaining cucumber halves. Tear crushed cucumbers into chunks.
Whisk together oil, vinegar, shallot, and mustard in a serving bowl. Season to taste with salt and pepper. Fold in crushed cucumbers, butter beans, and tarragon. Season to taste with salt and pepper, and serve.