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Even simpler, advertise your onion domain by including a Onion-Location header in the clear-net site’s response.

The Tor Browser automatically detects and offers to redirect you there. No need to memorize the onion domain.

This was the original purpose of the alt-svc[1] header, although it's rarely used for that purpose and instead used for upgrading clients to QUIC.

That sounds like a pretty nice place to add malware to a site once it's hacked and avoid detection.