'a lesson learnt' is correct and usable in written English.
You can use it to refer to something that someone has learned from an experience, usually a difficult or unpleasant experience. For example, "One of the hardest lessons I have learnt is that failure is often unavoidable."
"The team at SuperGurl will definitely be more careful with what we will put up in future as this is also a lesson learnt for us".
"That is a lesson learnt by the commission, that they have to be fairly careful because you can produce more damage," he said.
He upset an awful lot of natural Labour supporters by his actions and it's a lesson learnt by him and his aides.
Using a lesson learnt from history, Germany has been and still is "inspired by the determination to promote world peace as an equal partner in a united Europe", as stated in our basic law.
"There appears to be no real consent that has been given, particularly from younger people, and that could have a medium- to long-term adverse impact on community relations, which is a lesson learnt the hard way in England and Wales".
"It's a lesson learnt for all three of them.
4.12pm: Freedman says the Ministry of Defence was going to launch a "lessons learnt" exercise in 2003.
President Rajapaksa said the U.N.'s involvement was not needed: a "Lessons Learnt" commission, which he had appointed, would look into things instead.
For example, in 2010 it appointed a Lessons Learnt and Reconciliation Commission to investigate the final phase of the 26-year civil war.
(See 3.38pm) * The government was going to hold a "lessons learnt" exercise in 2003, but shelved that because the time was not deemed appropriate.
Rather than tackling these allegations head-on through a truth commission or criminal investigations, Sri Lanka created a "Lessons Learnt and Reconciliation Commission," whose mandate, composition and methods all cast serious doubt on its willingness to uncover what really happened in those fateful months.
