In the interests of pedantry, there are several ways. The simplest might by to bring the plane over land. There's also the option of going above the vertical limit of the controlled airspace, which seems to be debated, although it must be at least 30km, and less than the boundary with "space" which must be near the Kármán line at around 100km. A tunnel would be possible also, but prohibitively expensive. It would probably be cheaper and easier to get Hungary and Austria to leave the EU.

Barge up the Rhine