Popular with the price-is-no-object set, Opus is nonetheless low-key. Smooth, personable servers make you feel at home in the elegant renovated town house, which contains several small dining areas. The look is casually chic, and there's no shortage of suits. The menu changes every other month, and always features classic French as well as lightened-up dishes. Tuna tartare meets its match with black sesame seeds and lotus chips, and beef tenderloin with dauphinoise potatoes and rosemary-shallot jus is memorable. Desserts are easy on the eye and the palate. The wine list runs to volumes, with highlights of New and Old worlds; the knowledgeable staff can set you straight.