Thinly slice the fennel bulb and stalks into even pieces about 1/8 inch thick and set aside.

Roughly chop the fennel fronds for garnish and core the apple before slicing it into thin wedges.

In a dry skillet over medium heat, toast the walnuts for about 4–5 minutes until fragrant and golden brown.

In a large bowl, combine arugula, sliced fennel, apple wedges, and toasted walnuts. Gently toss the ingredients together.

In a small bowl, whisk together olive oil and a splash of lemon juice or vinegar until well combined.

Drizzle the dressing over the salad mixture and gently toss again to ensure everything is coated.

Serve the salad immediately on chilled plates or in a serving bowl, garnishing with reserved fennel fronds.