Combine the dill with the lemon zest and lemon juice in a bowl and slowly whisk-in the olive oil. Season the vinaigrette to taste with sea salt and freshly ground black pepper.
3 tbsp lemon juice, 3 tbsp extra-virgin olive oil, ¾ cup fresh dill, ¼ tsp lemon zest, Sea salt and freshly ground black pepper to taste
Combine the cucumbers, fennel and onion in a bowl and pour the vinaigrette over.