In a large saucepan, heat butter and oil over medium-high heat. Add onion; cook and stir until tender. Add asparagus and potatoes; cook 3 minutes longer. Stir in broth, lemon zest and seasonings. Bring to a boil. Reduce heat; simmer, covered, until potatoes are tender, 15-20 minutes., Cool slightly. Process soup in batches in a blender until smooth. Return all to pan and heat through. Combine garnish ingredients; serve with soup.