With a POD at the 11th century(when they came about), make the Hapsburgs control all of Europe for at least 50 years. I won't stress on any kingdom of Greater Britain being part of the Hapsburg monarchy.
A few lucky breaks could probably nail them England and Poland Lithuania.
From there some super lucky/capable monarchs can maybe bring in Russia. A Hapsburg Poland successfully interfering in an Alt Time of troubles to put a Hapsburg on the throne?
A Hapsburg England could help crush the Dutch revolt. From then on, have the Hapsburg emperor shore up his power in an alt thirty years war, making his power felt over all the principalities of Germany.
French get maybe brought in by some fortunate dynastics... Somehow.
I cant see the Russians allowing a catholic monarch to rule their orthordox country
There always is the option of conversion, and of course the Great Schism was 11th century, so around the PoD, if we can remove that, it becomes a lot easier.
Under one monarch.Is it one Habsburg or the house of Habsburg through several Habsburg kings ?
If you consider things in the late 17th or in the 18th century, most ruling families in Europe had Habsburg blood, though mainly throug female ancestors.
If you want one single Habsburg ruler, you can multiply the scenarios like what happened with Castile-Aragon : accident death bring a habsburg heir on the throne. But this will not work in countries of Salic law where you will need a total extinction of the national dynasty to have some kind of election of a Habsburg king.
But it will take more than 50 years to reach this result and your growing habsburg empire will have crumbled before it has reached the extent you're aiming at.