1. Preheat the oven to 375°F (190°C).
2. Clean the mushrooms and carefully remove the stems.
3. Chop the fresh spinach into small pieces.
4. Crumble the goat cheese into a mixing bowl.
5. Chop the fresh cranberries into small pieces.
6. If using, chop walnuts or pecans into small pieces.
7. In a bowl, combine goat cheese, chopped spinach, cranberries, and optional nuts.
8. Mix the stuffing ingredients until well combined.
9. Fill each mushroom cap with the stuffing mixture.
10. Arrange the stuffed mushrooms on a baking sheet.
11. Bake the mushrooms for 15-20 minutes or until mushrooms are tender.
12. While baking, prepare the balsamic glaze by combining balsamic vinegar and honey in a small saucepan.
13. Simmer the glaze over low heat until it thickens slightly.
14. Remove the mushrooms from the oven once cooked.
15. Drizzle the balsamic glaze over the stuffed mushrooms before serving.
16. Arrange on a serving platter and serve warm.
17. Enjoy this flavorful and elegant appetizer!