When that block is full of transactions, it is hashed.
This is not necessarily true. The block can be hashed even when empty or nearly empty.
Miners immediately start to search for a valid hash, even before the block has any transactions. (I guess, the incentive to mine a new block is much larger than to collect the transaction fees!!)
This was puzzling me too, but then I
understood somewhat more, thanks to @DannyHamilton.
Thank you butka. You showed me a very good post.
I will review my explanation later. This is more complicated than I thought.