You ask the waiter to bring you the mackerel, but without the teriyaki glaze. He says, "the menu says no substitutions, I'm sorry."
There's absolutely nothing wrong with running an establishment around the idea that it is what it is, here it is, you can have it if you want to buy it.
You ask the waiter to bring you the mackerel, but without the teriyaki glaze. He says, "Certainly. Is there anything else I can offer to make it even more to your liking?"
Again, that's a fine strategy. It recognizes that eating out is a choice, and that this establishment is in the business of treating different people differently.
Do you know what's not okay? "Well, we don't like to do this, but just this one time, I'll ask the chef, but I hope it's the only thing you want changed."