This reminds me of an episode of some old sitcom which featured a couple of people in a fancy restaurant. I don’t recall the show or the characters, but I do recall that one of them was a curmudgeon-type who didn’t dig upscale eats.
He went in there and just told the waiter, “I want a Porterhouse steak with fries.” The waiter looked at him like he was loony, but went back and that’s exactly what he got.
If you want something specific but it’s not on the menu, order it anyway. Any restaurant worth its salt - even a fancy one - will make you what you want if they can.
You may risk strange looks from the wait staff, and who knows what they’ll charge you, but it doesn’t hurt to ask.
So, you go into a fancy restaurant and want a turky sandwich, just say what you want and how. Even if there is not turky sandwich at all (let alone one with avocado slices and pesto garic mayonaise or other such evil things), if they have the raw materials, they should hook you up.
This reminds me very little of a favorite cartoon panel depicting a waiter in a very fancy establishment uncovering a smouldering, unrecognizable heap under the nose of a consternated customer. The waiter is saying:
“It’s a fried telephone book. We gave it a fancy French name and you ordered it!”
As Satan said, most reasonable restaurants will prepare food to your liking. That’s what they’re there for.
Why would you go to a fancy restaurant in the first place to get a plain turkey sandwich, anyway? From your OP, it sound like it wasn’t your choice, but I sure as hell wouldn’t go to Chez Paul to chew on their Buffalo wings or Chicago-style hot dogs. That’s not their specialty. There are plenty of restaurants who serve “comfort food”: meatloaf, sandwiches, casseroles, etc. It’s actually part of a growing trend among the restaurant business. That’s where you go where you wanna get food that tastes like your simple homemade, honest-to-goodness supper that your ma used to make.
Fancy food can be great. It’s not for everyones taste. I, for instance, love Indian curries (not fancy food by any stretch); many people can’t stand them. Somebody mentioned cilantro as a “trendy” ingredient. I read somewhere that it is the most used herb in the world. I love it. Some of my friends detest it. What one calls a “frou-frou strange sauce” might taste pretty damned good to me.
You should know what you’re getting yourself into, and if you don’t like that style of food, simply go somewhere else. What’s so difficult about that? Balsamic on a turkey sandwich sounds like a fine idea to me.