Melt 2 tablespoons butter and oil in a medium size saucepan at low heat. Add onion and leek and cook until tender. Add asparagus and season to taste. Cook stirring for 5 minutes. Add chicken broth and simmer, covered, for 20 minutes or until asparagus are very tender. Remove from fire and cool for 10 minutes.
Blend mixture until creamy and soft. Return cream to pan and add milk or cream. Correct seasoning. If soup is too thick add more broth to the desired consistency. Bring soup to boil, whisk in remaining butter and add lemon drops.
Cook asparagus tip for decoration in salted water for 5 minutes or until tender. Drain and reserve.
Serve soup in bowls and decorate with asparagus tips.