About 6-8 years ago, McDonald's had a line of like, "gourmet" burgers that came with grilled onions on them and big leafs of Romaine lettuce, those things were delicious but they discontinued them after only like, six months.
I’m old enough to remember the first “adult” burger McDonald’s launched, the Arch Deluxe. It came on a potato bun and had a stone ground mustard that was delicious. They had a huge marketing push for it, but then gave up before it had a chance to really catch on. I thought it was really good (for McDonald’s), but they didn’t really give it time.