Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a shallow bowl, combine the chopped walnuts, breadcrumbs, garlic powder, onion powder, salt, and black pepper.
In another bowl, whisk together the egg and Dijon mustard until well combined.
Dip each chicken breast into the egg mixture, allowing any excess to drip off, then coat with the walnut mixture, pressing gently to adhere.
Place the coated chicken breasts on the prepared baking sheet. Drizzle olive oil over the top of each piece.
Bake in the preheated oven for 20-25 minutes, or until the chicken is cooked through and the coating is golden brown.
While the chicken is baking, prepare the fennel slaw. In a large bowl, combine the sliced fennel, shredded carrots, and red onion.
In a small bowl, whisk together the apple cider vinegar, honey, salt, and pepper. Pour the dressing over the slaw and toss to combine.
Once the chicken is done, let it rest for a few minutes before slicing. Serve the walnut-crusted chicken alongside the fennel slaw.