1In 8-inch skillet, mix lemon juice, honey, salt and pepper. Cook over medium heat, beating occasionally with wire whisk, until honey is dissolved. Remove from heat; beat in olive oil. Taste and adjust the seasoning. Let cool to room temperature.
2Cut radicchio through the core into quarters; remove core. Cut radicchio crosswise into thin ribbons and place in large bowl. Remove thick stems from watercress; mix watercress with radicchio. Cut apple in half lengthwise; remove core. Cut apple crosswise into paper-thin slices (a vegetable slicer is handy for this). Add apple to radicchio and watercress. Toss with enough dressing to moisten salad.