About a decade ago at a job in Philly, we’d hunt down the spOt Burger cart (that’s how they capitalized it). Tiny little trailer/cart only big enough for one person to stand in, and this guy would park it somewhere new around center city/university city area every day. My memory is a little hazy so I might have some details wrong, but every day he’d grind a blend of ribeye and filet fresh to make the burgers in his cart, cooked around a medium, and served them on a brioche bun with pickled red cabbage and some other fixings. He got the fat content just perfect with the steak blend, and the toppings were unexpected but incredible together.
I haven’t been back in awhile but I heard he was opening a brick and mortar restaurant because his cart was so successful. Hope it’s true!