Place all ingredients for the balsamic chicken (through garlic) into insert of slow cooker. Cook on high for 4 hours or low for 6 hours. Remove chicken from insert and pour balsamic sauce into a small sauce pan over medium heat. Cook until reduced by half, about 10 to 15 minutes.
Meanwhile, preheat oven to 350ยบ F.
Break apart chicken into large chunks and mix together with cream cheese and about 1/4 cup of reduced balsamic sauce. Set aside.
Layer spinach, prepared balsamic chicken salad, and cheese on one half of croissant. Place into oven for 5 minutes or until cheese has begun to melt. Drizzle with more balsamic sauce, if desired. Remove from oven and top with other half of toasted croissant.
Notes
Store any remaining balsamic sauce in an airtight container in the refrigerator for up to a week.