How to stop mint flavor dying when cooking?
While it is true that the mint flavor will fade with cooking, it is still there to some degree.  I bet you would be able to identify the difference if you left it out.  However, whenever you want to highlight a fresh herb, such a mint, it is good practice to chop some of that herb at the last possible moment before serving, and garnish your finished product.  That will greatly enhance the flavor and the aroma.