For a great lunch spot in the Marais, look no further than Chez Janou. It's not undiscovered but when we were there, we were surrounded by French speaking, wine guzzling locals. And just look at the place - it's what you want Paris to look like, right?
We ordered mussels and white wine, and to finish it off - a bowl of chocolate mousse twice the size of our heads. It is the chocolate mousse of your dreams, like eating a smoothy, creamy, dark but sweet chocolate bar. They spoon out a big blob for each person. Don't put your greedy spoon directly in the bowl for "one last bite" (I would never do that) - they will yell at you. I think they take whatever is left and put it in the next bowl for the next customer.
Address: 2 rue Roger Verlomme, 3rd arr.
Metro: Chemin-Vert, Bréguet - Sabin, Bastille