What should I do when my work has been published by my supervisor without my knowledge?
There is probably little you can do formally or legally. Not citing your thesis was a lapse, but getting that changed is not likely to happen. You can, of course, point people directly to your thesis. 

I don't see any ethical issues with the publication itself. The work was, in some sense, unfinished. It is natural to want to finish it and you weren't there to help. But you should have been cited. But the ethical lapse was in failure to cite, not the publication itself, assuming that it was an extension. 

But you might also be able to obtain a letter from your old supervisor explaining that the publication in question incorporated prior work of yours. That might be enough for your current purposes. 

However, you might also be able to publish your thesis unchanged with original dates attached. It would be a late publication but if you can convince a journal to publish it, you could establish priority on at least some of the ideas. Your university will verify that the work was done prior to the new publication's date. If you change/update it, however, you lose the advantage of priority. 

But try to work first with your old supervisor. 

Another option is to extend not only your own old work but the publication itself. Then it is new work. You can cite your old thesis and the newer publication in this work. Ultimately that is what you want to be able to do, of course.