Preheat the grill or grill pan to medium-high heat.
Grill the chicken breast for about 6-7 minutes on each side until fully cooked and juices run clear. Remove from the grill and let it rest for a few minutes before shredding.
In a mixing bowl, whisk together the mayonnaise, Greek yogurt, apple cider vinegar, honey, poppy seeds, and a pinch of salt and pepper until smooth. Adjust seasoning to taste.
In a large salad bowl or serving platter, layer the baby spinach and mixed salad greens as the base.
Top the greens with the shredded chicken, strawberries, blueberries, and raspberries.
Drizzle the prepared poppy seed dressing over the salad and toss gently to combine all ingredients.
If desired, sprinkle sliced almonds and feta cheese on top before serving.
Serve immediately and enjoy your refreshing Summer Berry Chicken Salad!