Melt the butter in a Dutch oven or large pot over medium low heat. When the butter is sizzling add the chopped onion. Cook the onion over medium low heat for 10 minutes, stirring occasionally. By cooking the onion slowly you allow the natural sugar in the onion to begin to caramelize, which results in a richer, deeper flavor.
When the onions are translucent and have begun to brown very slightly add the garlic and sun dried tomatoes. Stir the mixture occasionally and saute an additional 3-4 minutes.
Increase the heat to medium and pour in the dry white wine. Use a spatula to scrape up any bits stuck to the bottom of the pan. If you prefer not to use the wine you can substitute chicken stock.
Next, add the cooked, shredded chicken, spices and broth. Increase the temperature to medium-high heat and bring to soup to a simmer, uncovered for 10 minutes.
Remove the soup from heat and pour in the heavy cream, grated parmesan cheese and the chopped spinach.
Stir the soup until the spinach wilts.
