Reheat chicken until warm. Slice chicken into ½-inch thick pieces and set aside.
Cut tomato into ¼-inch thick slices and season with salt and pepper to taste, set aside.
Remove skin and pit from avocado. In a small bowl, mash avocado with salt and pepper to taste. Set aside.
In a damp paper towel, wrap pita bread and microwave for about 10-15 seconds to soften.
Remove pita from paper towel and cut along the edge to separate the top and bottom.
Fill each pita with half the avocado spread, 2-3 slices of tomato, half the spinach, and chicken.