Apparently, if you’re looking for Lent and fast-day food in oldish French cookbooks, you look for a “meagre dish” (plat maigre). They are either meatless, or meat- and dairyless. These dishes are eaten on a “meagre day” (jour maigre) when you abstain from meat (faire maigre). Things are complicated, though, by the existence of a fish which the French today call “maigre”, and by using that word just to mean “lean”.
The word for Lent is Carême. If you see non-Lent-looking food for “Mi-Carême”, that’s for Thursday in the Third Week of Lent (Mid-Lent, in this French expression), when the French have a big party to celebrate getting halfway over the Lenten hill, so to speak. There was also a famous olden days chef named Careme, so be careful! You could think all sorts of wrong things about what the French ate for Lent!
I’m sure a lot of you folks know this stuff already, but I’m hard-pressed to find useful Catholic search terms, even in languages I’m familiar with! (French and I are practically strangers.)