This is a video of Natalie Sideserf of Sideserf Cake Studio (previously: her realistic pizza slice and Garbage Pail Kids Adam Bomb cakes) providing a step-by-step tutorial for how to make a cake that looks identical to an onion, complete with papery exterior (she uses edible wafer paper for that part). Impressive. And she's so good at making these tutorials that she almost had me convinced I could actually do this before I stopped for a second and realized there's no way my onion cake would turn out looking like anything but a raging kitchen fire.
Keep going for the whole video.
Thanks to Madz, who agrees that's an awful lot of work just to keep a roommate from eating your cake.