Photoshop: Frame Animation Has No Onion Skin Setting?

I am in frame animation mode and unwilling to change to video animation. I want to be able to see the previous frame with lowered opacity and at the same time drawing a new frame. This is especially important to me because I am hand drawing with a tablet. I know that there is is onion skin on video timeline but that mode would restrict my work. Is there a setting or perhaps a plugin that will allow me to enable onion skin on frame animation.
Thanks