In a small bowl or jar, whisk together mayonnaise, apple cider vinegar, honey/maple syrup, milk, poppy seeds, salt, and pepper until smooth.
Wash and dry the spinach. Thinly slice apples and onion. Toast pecans in a dry skillet for 4-5 minutes, if desired. Toss apples in lemon juice to prevent browning.
In a large bowl, combine spinach, apples, cranberries, pecans, onion, and cheese if using. Add grilled chicken for a more filling meal.
Drizzle dressing over the salad and gently toss to coat or store separately if making ahead.
Serve immediately for best texture and flavor.