Combine chicken broth, milk, and heavy cream in a large pot and bring to a boil. Stir in polenta and reduce heat to low. Cover and simmer 20 minutes, stirring occasionally.
While the polenta is simmering, cook the bacon in a large skillet until done. Remove bacon from the pan and set aside. Leave about 2 tablespoons of bacon grease in the pan. Sautee the onions until soft and golden, add the spinach and cook until wilted.
When the polenta is finished cooking stir in butter and 1 cup of parmesan cheese. Add salt and pepper to taste.
Stir in the onion-spinach mixture and mix well. Pour into a greased 9x13 baking dish. Top with remaining 1/2 cup of parmesan cheese and bacon.
Bake 10-12 minutes until hot and bubbly. Garnish with spinach leaves and serve immediately.