Stuffed Portobello with Balsamic Reduction

INGREDIENTS

• 1 10-ounce package frozen spinach
• 2 tablespoons olive oil
• 1 cup chopped sweet onion
• 3 garlic cloves, minced
• 1/4 cup sundried tomatoes, chopped
• 1/4 cup marinated artichoke hearts, drained and chopped
• 1/4 cup plus 6 tablespoons finely grated Parmesan cheese
• 1 5-ounce package soft fresh goat cheese, crumbled
• 1/4 cup unseasoned dry breadcrumbs
• salt and pepper, to taste
• 4-6 portobello mushroom caps
• 1/2 cup balsamic vinegar
• 2 tablespoons brown sugar
• 1 sprig fresh thyme