Techniques to toast mate tea leaves
There are green teas in China that have a 'toasty' flavour and that is usually achieved through pan frying. I would suggest to try that in a pan at home yourself (without oil). Make sure the leaves are continuously stirred, or they will burn. 

Check for instance this video (at 25 sec) for a demonstration:
https://www.youtube.com/watch?v=tmKkVLadXKQ