For the dressing: Place the mayonnaise, Greek yogurt, milk, spinach, basil, parsley, garlic and lemon zest in a blender with a pinch of salt and pepper. Blend until smooth, then taste and adjust seasoning as needed. Set aside.
For the salad: Add the chicken, grapes, walnuts, scallions and cucumber to a large bowl. Pour the dressing over the salad ingredients and toss to combine. Transfer to a serving bowl and garnish with the basil and parsley. Serve on a bed of lettuce, in a sandwich or wrap, or cover and chill until needed.