Grasp the mettle/nettle

Is the phrase grasp the mettle or grasp the nettle?

It's "grasp the nettle". The point is that if you just brush a nettle leaf tentatively it will sting you, whereas if you grip it firmly it won't. (This is true of the European nettle - I have tried it. I don't know about American varieties). Therefore, to "grasp the nettle" means to take bold, decisive action to deal with a difficulty.