Melt butter in a soup pot over medium heat. Add minced onion and salt; cook and stir until onion is soft and golden, about 10 minutes.
Pour chicken broth and water into pot. Bring to a boil, reduce heat to medium, and simmer 15 minutes.
Place asparagus pieces in simmering broth. Cook, stirring occasionally until asparagus is just tender and still bright green, 5 to 10 minutes. Remove from heat.
Use an immersion blender to blend soup until smooth, about 3 minutes.
For a finer texture, strain soup through mesh strainer. Return strained soup to pot and place on stove.
Turn heat to low. Season with cayenne pepper, black pepper, and nutmeg. Adjust salt to taste. Whisk 1/2 cup cream into soup.
Place 1/4 cup cream, Parmigiano-Reggiano cheese, and lemon zest in a small bowl. Whisk until frothy and slightly thickened, about 1 minute.
Ladle soup into bowls. Drizzle a spoonful of Parmigiano-Reggiano-lemon cream on top of each serving and swirl to form a random design.
Sprinkle with a pinch of lemon zest and cayenne pepper, if desired.
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