This bright and peppery relish is a perfect counterpart to fried entrées like this crispy Chicken Schnitzel with Dill and Sesame.
Slice peel and pith from lemon with a sharp paring knife, removing as little flesh as possible; reserve peel. Working over a large bowl (to catch any juices), cut away segments from between membranes. Cut supremes into thirds and reserve in a small bowl; tip any juices on cutting board into large bowl. Squeeze membrane and peel over large bowl to extract juices.
Whisk Dijon into 2 tablespoons lemon juice in large bowl; season generously with salt and pepper. Slowly whisk in oil. Add watercress, apple, and fennel, tossing to evenly coat. Stir in capers and lemon pieces. Relish is best served the day it's made, but can be refrigerated in an airtight container up to 1 day.