You mean "build a sandwich" joints, right?

Because only one fast food burger or chicken place that I've gone to and that I consider "fast food" seems to have this option. Harvey's... and they're silly expensive where I live compared to the others.

I could be living in a place where it's simply not a thing and in the submarine sandwich places it could be. IDK. Would like to hear others' opinions on this.

Maybe a LPT is: Buy your own salad-type fixin's, take your fast food home, and then add them for some nutrition there.