Dama Bianca (Fennel and Celery Salad)

2 medium fennel bulbs, stalks discarded
6 pale inner (white) celery stalks, leaves discarded and stalks thinly
sliced
1 (1/2-lb) ball buffalo mozzarella (optional), roughly torn
1/2 tablespoon grated lemon zest
2 tablespoons fresh lemon juice
6 tablespoons good-quality fruity extra-virgin olive oil
1/4 tsp. fine sea salt
Halve fennel lengthwise, then thinly slice crosswise about 1/4 inch
thick.  Toss with celery and arrange on a platter with mozzarella.
Whisk the sauce together and drizzle over the salad.