Preheat your oven to 375°F (190°C) and toast the walnuts lightly if not already toasted; then, finely chop them in a food processor or with a sharp knife.
Pat the chicken breasts dry with paper towels, then season them with salt and pepper on both sides.
Place the chopped walnuts on a plate. Coat each chicken breast thoroughly with the chopped walnuts, pressing gently to help the nuts adhere evenly.
Heat a tablespoon of olive oil in a skillet over medium heat. Once shimmering, add the chicken breasts and cook for about 4-5 minutes per side until golden brown and cooked through.
While the chicken cooks, thinly slice the fennel bulb using a sharp knife or mandoline for uniform ribbons.
In a mixing bowl, whisk together the apple cider vinegar, a tablespoon of olive oil, honey (if using), and a pinch of salt and pepper to make the dressing.
Pour the dressing over the sliced fennel and toss gently until all the slices are coated and slightly wilted, creating a crisp, tangy slaw.
Arrange the toasted walnut-crusted chicken on plates and top with a generous serving of fennel slaw.
Finish with an additional drizzle of olive oil or a squeeze of lemon if desired, and serve immediately.
