In a bowl, whisk together all dressing ingredients with a hand-whisk until thoroughly combined. Transfer to a dressing container; chill until ready to use (can be made ahead of time.)
Grease outdoor grill or indoor grill pan. Season chicken with a thin layer of kosher salt and freshly ground black pepper on both sides, pressing to adhere. Grill chicken just until nice grill marks appear and no longer pink in center; do not over cook and remove immediately from heat once cooked. Let chicken rest at room temp at least 5 minutes; then slice into strips.
In a large serving bowl, combine spinach leaves, sliced strawberries, feta cheese, and avocado slices. Pour desired amount of dressing onto salad and gently toss to combine. Divide salad into individual serving plates, topped with sliced chicken and candied nuts. Serve immediately.