Melt the butter in a stockpot and add the onion.
Sauté the onion until browned, then stir in the garlic and cook 1 minute more.
Stir in the asparagus and mushrooms, then sauté for 4 minutes.
Pour in the vegetable broth along with the spinach and tarragon.
Bring to a boil, then reduce heat and simmer for 30 minutes on medium- low heat.
Remove from heat, then stir in the cream, lemon juice, and parsley.
Cover and let rest for 20 minutes, then season with salt and pepper to taste.