Place the pesto, spinach and lemon juice in a food processor or blender and puree until combined and smooth. Add the mayonnaise and puree again for about 5 to 10 seconds to combine.
Pour the pesto mixture over the chicken, add the salt and pepper, and stir to completely coat all of the chicken. Serve on your croissants or your favorite bread. Leftovers can be stored in an airtight container in the refrigerator for up to 2 days.