The "Sandvich" is a heavy secondary item. Once equipped and injured, left-click/primary fire to eat it, or Right-click to throw it on the ground for as a health pickup for anyone. The Sandvich has a 30 second cooldown, but will be restored instantly by running over a health pickup while at full health.
Any heavy without a medic should love his sandvich dearly, so this can come through normal play. Just eat one anytime you are injured and there's a break in fire.
The Sandvich is awarded for completing ten heavy achievements. It can be crafted with a heavy token+Secondary token+scrap metal, as one of five possible results. You can also get it as a random drop or on the steam market. I currently see "vintage sandvich" at 12 cents USA.
I don't know if the robo-sandvich, Buffalo steak sandvich, or other times count. Vintage, festive, and strange sandviches do count.