I think the 'nature' part refers to the fact that there is no caramel in this recipe.
I think you could just say "custard" or "egg custard"

Never mind what I said. I checked with my husband and he says that the 'nature' refers to a lack of fruit or other flavorings, e.g. flan aux fruite, etc. In Amercian English you could say flan or crème caramèle (I looked in the OED and Flan in British English is not the same thing.)
I think the above answer "simple caramelized custard" is probably your best bet.