1. Preheat the grill to medium-high heat.
2. Season the chicken breasts with salt and pepper.
3. Grill the chicken for about 5-7 minutes on each side, or until fully cooked.
4. Let the chicken rest for a few minutes, then slice it.
5. In a large mixing bowl, combine the mixed greens, strawberries, blueberries, feta cheese, and walnuts.
6. In a small bowl, whisk together the olive oil, balsamic vinegar, and honey.
7. Drizzle the dressing over the salad and toss to combine.
8. Top the salad with the sliced grilled chicken and serve.