I'm from western Switzerland, where the Gruyère cheese (Swiss AOP) comes from. Making a sandwich or an open-sandwich with Gruyère and mustard is something quiet common, that a lot of people enjoy for it's simplicity.

I don't know if the version with tomatoes, specifically, is popular, but it is in my family for sure!

These are my thoughts exactly. This feels like a French thing that I've never been exposed to. I'm now curious about this from many nations. What is the everyday lunch that is not the globally recognized stereotype national dish? As an American I feel like it would be something lame like peanut butter & jelly.

That's the power of one-side toasted shokupan: crispy on one side, dense and fluffy on the other!