In PKI, with a hash-encrypted message (first hashed, then encrypted), how is the plaintext ever retrieved? Consider a normal PKI encryption flow - hash the plaintext, then encrypt the hash with the private key of the sender and then encrypt it with the public key of the recipient. All this is good - and I am good even with the decryption steps taken by the recipient.  

I still don't understand how the recipient gets the plaintext from the hashed ciphertext. The recipient can easily 'decrypt the hashed plaintext'. But then what - all you are left with is a hashed plaintext. There is no way to un-hash it. How does the recipient retrieve the plain text message from the hashed message?
Signature generation is not encryption with the private key. Still, the basic flow of what you describe is correct for signature generation. However, the verification step is where everything derails.
As indicated in the comments, it is impossible to reverse a cryptographic hash. However, for signature verification, it is assumed that the verifying party knows the signed message M. The verifying party can then re-calculate the hash and use that as input for the rest of the verification procedure. So if the verifying party doesn't have the message M then you simply need to send it together with the signature.

See for instance step 2 in RSA signature verification specified for the PSS signature scheme:

Input:
      M      message to be verified, an octet string
      ...

and then later:

 ...
 2. Let mHash = Hash(M), an octet string of length hLen.
 ...


The verification procedure may indeed involve retrieving the original hash and perform comparison. However, other signature verification functions such as the verification function of ECDSA may simply require the re-calculated hash as input for the verification function. So in that case the original message hash isn't even retrieved from the signature.

There are schemes that perform message recovery. A few of those schemes are outlined in ISO/IEC 9796-2: Digital signature schemes giving message recovery. However, they are not often used anymore. In that case the message itself - or at least part of it - is included with the hash, the message is still not retrieved from the hash.

Remember that signatures do not provide message confidentiality. If you also want to keep your data secret then you should encrypt the data and signature over the data.