Trim the stalks and fronds from the fennel bulb, saving a few fronds for garnish. Cut the bulb in half and cut out the tough core.

Using a mandoline or a sharp knife, shave the fennel into slices about 2mm thick. Place in a large mixing bowl.

Core the apple and slice it just as thin. Toss the apple slices with 1 tbsp of the lemon juice right away to stop browning, then add to the bowl with the fennel.

In a small mixing bowl, whisk the remaining 2 tbsp lemon juice with the olive oil, Dijon mustard, honey, salt, and pepper until the dressing looks smooth and slightly thickened.

Toast the walnuts in a dry 8-inch skillet over medium heat for 3 to 4 minutes, shaking the pan often, until fragrant. Let cool slightly, then roughly chop.

Pour the dressing over the fennel and apple. Toss gently with salad tongs until every slice is coated.

Scatter the shaved parmesan, toasted walnuts, and chopped parsley or mint over the top. Add the reserved fennel fronds.

Serve right away while the fennel and apple are still crisp.