Roll the cucumber pieces up in a clean linen napkin or towel, and then smash them, pressing down hard with the heels of your hands while ensuring the towel doesn't unroll, to release the juices. Transfer the crushed cucumbers to a cutting board, and then coarsely chop. Combine the cucumbers, sliced fennel, dates, walnuts, chives and some of the fennel fronds in a salad bowl and mix gently.
Dress the cucumber salad with the lemon juice and 4 tablespoons of fruity olive oil. Then season with sea salt and pepper. Toss the salad lightly, garnish with fennel fronds and serve.