It seemed like a crazy idea in the late "80s--serving real French food on a remote corner of Tenth Avenue--but this place did right by classics like cassoulet and cervelle (brains) in red-wine butter. When chef-owner Jean-François Fraysse opened Quercy in Brooklyn, however, the kitchen here lost some sharpness. Anything with the excellent creamy vinaigrette is still worth ordering, but on a recent night, the leek salad"s lentils were undercooked, and a perfectly seasoned foie gras terrine was overburdened by jellied fat. Meaty lamb sausage with sautéed apples was great, yet the potato-gratin side had spent too much time in the oven. There"s nothing wrong with the sumptuous chocolate cake, which arrives drowning in a pool of chocolate sauce.