Cut cucumber in half, and slice crosswise into 1/4-inch-thick slices. Process 3/4 cup cucumber and lemon juice in blender. Add olive oil in a thin stream; pulse until combined. Season the dressing with black pepper, to taste; transfer to a large bowl. Add remaining 1 cup cucumber, watercress, artichoke hearts, celery, red onion, and feta cheese to a bowl. Toss with the dressing, and divide among 4 bowls; serve.