From lemon, grate 1/2 teaspoon peel and squeeze 2 tablespoons juice. In jar or container with tight-fitting lid, combine oil, vinegar, lemon peel and juice, 1/4 teaspoon salt, and 1/2 teaspoon freshly ground black pepper. Shake well. Dressing can be refrigerated up to 2 days.
Pluck 2 tablespoons fennel fronds from fennel tops and reserve for garnish. Trim and core fennel. With adjustable-blade slicer or very sharp knife, very thinly slice fennel. With vegetable peeler, peel alternating strips from cucumber skin, then thinly slice cucumber crosswise at an angle.
On large serving platter, layer fennel, cucumber, and apple. Shake dressing again and drizzle all over. Top salad with pomegranate seeds and fennel fronds. Sprinkle 1/4 teaspoon salt all over.
Recipe Notes
If you buy fennel bulbs without their tops, you can use 2 tablespoons fresh dill instead.