I'm not quite sure how to correct it though. How about these possibilities:
Get free hash browns... (maybe implies an unlimited amount)
Get a free hash brown...(can hash browns be singular?)
Get a free order/side of hash browns...(the best option in my book)
The more I type this, the more I realize "hash browns" is a really weird name...
I need to quit obsessing about corporate grammar and get back to work!