Step 1Saute 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 2Transfer 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.
Step 1Dredge asparagus in flour, dip in beaten egg and roll in bread crumbs. Heat enough oil to cover asparagus in skillet over medium heat. Add asparagus and fry until golden brown, 1 to 2 minutes. Remove with spatula and drain on paper towel. Cut spears on bias into thirds.