Preheat the grill to medium-high heat.
In a small bowl, whisk together honey, Dijon mustard, olive oil, garlic powder, onion powder, salt, and pepper until well combined.
Place the chicken breasts in a shallow dish and pour half of the honey mustard mixture over them, reserving the other half for later. Marinate for at least 15 minutes.
Grill the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F. Brush with the reserved honey mustard mixture during the last few minutes of grilling.
While the chicken is grilling, prepare the salad. In a large bowl, combine mixed greens, sliced peaches, feta cheese, and walnuts (if using).
Drizzle balsamic vinaigrette over the salad and toss gently to combine.
Once the chicken is cooked, let it rest for 5 minutes before slicing. Serve the sliced chicken on top of the peach salad.