1957 German Grand Prix

Notably this was on the Nordschleife, an ~ 23 km track (most modern F1 tracks are around 5 km). F1 stopped using the Nordschleife portion of the Nurburgring in 1976 because it’s considered too dangerous and it’s extreme length makes it difficult to adequately staff all portions of the track and be able to respond to emergencies quickly.

The Wikipedia mentions that they had F2 cars also racing at the same time. Again, this is likely due to the Nordschleife‘s length (longer track means you need more cars for viewers to watch. Otherwise they are stating at an empty track for most of the time).

It is unfortunate that we will probably never see a modern Formula 1 car racing hard at the Nordschleife again.