French-style potatoes? Not french fries?

I was in Paris and had a delicious meal at a non-touristy bistro that included potatoes cut into half-inch circles, cooked in what seemed to be butter so they were crispy on the outside and soft inside, and served with dijon mustard. Came with the duck I ordered, which also seemed to be cooked in butter after being broiled. Any recipes for potatoes like these? Is there a name for them?