Shred the rotisserie chicken into bite-sized pieces, ensuring the skin and bones are removed.

Dice the red onion and celery into uniform 1/4-inch pieces.

Chop the baby spinach into smaller pieces.

In a large mixing bowl, whisk together mayonnaise and Greek yogurt until smooth. Season with salt and black pepper.

Fold in the shredded chicken, diced vegetables, and chopped spinach into the dressing.

Adjust seasoning, adding more salt or pepper if needed. Thin the mixture with water or lemon juice if too thick.

Serve or chill in the refrigerator for at least one hour before serving.