Bring a large saucepan of lightly salted water to a boil. Add the asparagus and spring onions; cook 5 to 7 minutes or until asparagus is tender. Strain through a mesh strainer, and press out excess water. Place into the bowl of a blender and puree until smooth, then pour into a bowl and set aside.
Rinse out the blender, then add the egg yolks, lemon juice, salt, and Tabasco sauce. Cover, leaving the hole in the lid open and blend for about 5 seconds. Continue to blend at high speed while pouring the butter in a thin stream though the hole in the lid. Turn the blender off once all the butter has been added and the sauce has thickened. Pour into the asparagus puree and fold until evenly blended. Serve immediately.