I believe what you're looking for is the ironically named "beefsteak burger", which is actually just a giant tomato of the "beefsteak tomato" variety. (Real)
Discussion
Another whole food option is Portabella caps.
Now, mushrooms are mostly water, and these can make for a pretty soggy burger. So best recipe is to sauté upside down with marinade in the cap and a heavy weight on top. I use a cast iron griddle, to press the water out while it cooks.
Make a double burger with all the fixings, that any meat lover would enjoy. It’s not a meat substitute. It’s a mushroom burger. Yummy on its own right.