Nice, cozy spot for warm comfort food on a cold night. The highlight is any of their variations on shepherd's pie (pâté chinois): my childhood memories of this dish are boring, bland, stodgy, and definitely not something I would order in a restaurant. Forget that: at Mâche!, they somehow make it delicious. And the spicy one is anything but bland. Highly recommended.
I've also had the macaroni and cheese: unremarkable. You can definitely find better versions of this.
Another bonus: the rest of the world hasn't found it yet, so Mâche! is one of the few places on St-Denis with available tables and good food.