How do you get your hand on such watch-only wallet?
It's only possible if you've imported a "master public key" from a standard wallet.

The bad news is: there's no other way to reverse the transaction made by sweeping your paper wallet's key, unless the transaction is still unconfirmed and it has RBF flag.
If it's unconfirmed, you need to act fast, import the private key to a new wallet using "File->New/Restore", set a wallet name, next select "Import bitcoin addresses or private keys", them paste your private key.
The transaction should appear there, right click on it and select "cancel transaction" and sign then broadcast to send it back to yourself.
Note that the transaction should be unconfirmed and flagged as "replaceable" (RBF: True).