I'm surprised that you didn't know that HORNET is a source routed onion relaying protocol. https://scion-architecture.net/pdf/2015-HORNET.pdf

High-speed Onion Routing at the NETwork layer.

It was published in 2015 and two years later Osuntukun, Antonopolous and Pickhardt made LN.

I know about it because it was the second thing about LN that inspired me to start working on Indra, source onion routing is the basis of Indra, the secret sauce is the use of ECDH shared secrets to create bidirectional paths controlled by the client.