The phrase "lesson learnt" is commonly used, especially in British English, and it is considered correct. However, in American English, "lesson learned" is more frequently used. You can rephrase it in various ways, such as:
- "Lesson understood"
- "Lesson acquired"
- "Takeaway"
- "What I learned"
- "Insight gained"
Each of these alternatives conveys a similar meaning but may fit different contexts better.