Pour stock into large frying pan. Add garlic, rosemary and bay leaf. Bring to simmer.
Prepare asparagus by holding each spear at its end and bending until it snaps. Discard tough bottom part. Set spears in frying pan and cover with onions. Partially cover pan. Simmer 4 minutes.
Remove spears from stock and place on serving platter. Discard bay leaf. Strain stock into small bowl, reserving onions and garlic.
Place onions, garlic and parsley in blender or food processor. Add 1/3 cup reserved stock. Process until smooth. Drizzle sauce over warm asparagus.