I grew up eating slices of homemade bread and butter topped with all sorts of fresh veggies and salt. Mostly we sliced radishes or green peppers to top the bread. Occasionally we sliced our own homemade dill pickles as a topping. Recently I've read that this is very "French." I don't know the origin of these little sandwiches, but our home was Polish and German and all we knew was these were really good. Radishes alone can be a little strong but the bread, butter and salt mellow them right out. The green peppers are good with that fresh green pepper flavor that just needs just a little something to soften it a bit.
The bread must be homemade or some form of robust white or sourdough bread. I'm using sliced ciabatta here. I love ciabatta with all its holes and chewy texture. The butter is cold from the fridge to make thin smooth bits on the bread. I don't care for soft room-temperature butter. (My husband insists it has to be soft and "spreadable." We fought about this until we decided to keep mine in the fridge and his on the counter.)
Don't use much butter. It detracts from the veggies--we don't want this to be creamy. I tried fleur de sel on top but prefer smaller grind table salt. The crunchy salt crystals were too much on these sandwiches for me.