Grill the Chicken: Preheat your grill to medium heat. Rub the chicken breasts with olive oil and season with salt and pepper. Grill the chicken for 6-7 minutes on each side, or until fully cooked and the internal temperature reaches 165°F (75°C). Let the chicken rest for 5 minutes before slicing it thinly.
Prepare the Salad Ingredients: While the chicken is cooking, wash and dry the mixed greens. Place them in a large salad bowl. Add the fresh blackberries, crumbled feta, sliced almonds, and red onion (if using).
Make the Dressing: In a small bowl, whisk together the balsamic vinegar, honey, Dijon mustard, olive oil, salt, and pepper until the dressing is smooth and emulsified.
Assemble the Salad: Once the chicken has rested, slice it into thin strips. Add the sliced chicken on top of the salad. Drizzle the dressing over the salad and toss gently to combine, making sure all ingredients are coated.
Serve: Serve the salad immediately, or chill it for 10-15 minutes for a cooler dish.