Fudruckers. It's Fudruckers that has the Fixins Bar. And my gosh are those good burgers. I haven't had one in ages, but maybe one will open near here some day.
I'm literally salivating thinking about those burgers. A complete Pavlovian response.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Here, for example, is the quintessentially English cucumber sandwich
I, too, would call the extras which go on top of the meat/veggie pattie/burger inside a bun "toppings", not fillings.
It never occurred to me this would get so confusing!
It's not just burgers. A place like Subway makes a whole bunch of different kinds of submarine sandwiches. You order the basic (say turkey or Italian or ham) and then you tell them what you want on it.
I don't think we here think of a sandwich as a blank slate. It's something between bread which can optionally have something else on top of it.
Am I right in understanding though, that you do refer to a meat pattie inside a burger bun (with or without salad etc) as a "sandwich"?
Also, I just got done heating my soup at the microwave (at the coffee station) and the woman behind me started heating up a frozen (prepackaged) chicken pot pie. So, I guess adults do eat them here. I'd never have guessed that. Although in my defense, she often talks in a rather annoying baby voice.