In a large sauce pan, sauté onion in melted butter over medium heat until onion becomes translucent. Add flour and cook, stirring constantly. Whisk in stock; stir in broccoli and bring to a boil. Once boiling, reduce to simmer and cook, stirring as needed; 8 minutes.
Add entire bag of spinach to sauce pan and sauté until spinach is wilted and broccoli is tender.
Stir in cream. Using an immersion blender or standard blender, process soup until smooth.
Season with salt and pepper to taste.
Top with a broccoli floret and sprinkle of Parmesan cheese.