In between the bun-sized tomato halves, you'll find a burger patty, onion, lettuce, and sauce. The burger only comes in a combo with a Caprese salad cup for 880 yen (~$7.07 US).
It's only available at one location in Osaki, Tokyo from 2 PM to 5 PM through the end of August.
In case you're wondering how it tastes, the folks at Rocket News 24 tried it recently and took many-a-photo.
It should be noted that Mos Burger regularly offers a rice bun in addition to normal burger buns. I wonder if they'll do an onion bun burger next.
