I believe it has to do with the manufacturing process of the aspherical elements. Cheaper production processes tend to cause onion bokeh. Expensive ground aspherical elements tend not to onion. I wonder how much money Tamron saves on each lens through their methods.
not sure about this post.
on one hand, the tamron does seem sharper than the canon L mk1, on the other, onion bokeh might pop up when you least expect it. Maybe with the sharpness comes the onion? I dont know, just throwing stuff around.