Making meat tender?? Tips!?

In my experience, meat doesn't like swift changes of temperature, (which is why slow cooking and leaving meat to rest after cooking works wonders) so maybe frozen meat into hot oil would affect the tenderness. How long did you cook it for? I made a rendang once and the recipe said to leave it simmering for 2.5-3 hours, and it seemed to work a treat. (P.S. I use recipes from www.bbc.co.uk/food)