A tartine can be anything on top of a slice bread or toast, from your morning baguette smeared with butter (tartine beurre or tartine beurrée is bread and butter) and jam to elaborate savoury concoctions, like Italan bruschetta or an open faced sandwich. A spread on bread is a tartinade.
The expression ‘En mettre des tartines’ or ‘tartiner’ is to ‘over elaborate when writing,’ which of course I never do because it means the reader is going to lose interest as you bang on and on about something that seems really obvious from the get go and isn’t really necessary which reminds me of the story of James Baker the former US Secretary of State complaining that if Europe has a so-called ‘butter mountain’ how come restaurants in France never serve butter on the table like they do in the USA which of course the French response was our bread doesn’t need to be tartiner-ed unless of course you’re eating crudités or oysters or ham and maybe a few other things with which the French believe you should always have beurre demi-sel, but in general if you are like James Baker and maybe not even someone as important as a Secretary of State and ask for some butter s’il vous plaît in a restaurant you’ll probably get some.
Image by Conan from Livorno, Italy