Literally péché = sin and mignon = cute, so péché mignon = cute sin ? Kinda way to underrate a sin? It more or less means someone's weakness of something, especially food.
Fondant au chocolat (chocolate fondant) is one of my péché mignons. For my chéri, last night I made a Salmon cream tagliatelle (pasta is my péché mignon too) and chocolate fondant.
What's your péché mignon ?