Step 1 Saute onion and potato in butter in large saucepan over medium heat until onion is soft, 6 to 8 minutes. Add asparagus and saute until bright green in color, 3 to 4 minutes. Add enough water to reach level of 1/2- to 1 inch above asparagus. Bring to boil and simmer 20 minutes.
Step 2 Transfer potato-asparagus mixture to blender and puree in batches until smooth. Divide spinach and puree in batches with potato-asparagus puree until blended and smooth.
Step 3Strain soup through sieve. Return to saucepan and bring to boil. Reduce heat and simmer 5 to 10 minutes. Add salt, pepper and vinegar to taste. Garnish with fried asparagus and parsley.