Dinner-party elegance in a 20-minute dish? It's possible, when you start with great ingredients that require little embellishment. Here, we use sea bass, a tender, mild-flavored fish with a lovely buttery quality. There are several sustainable options to choose from (Atlantic black sea bass, for one), so ask your fishmonger to ensure you're making a responsible choice. You can also use cod, sablefish (also called black cod), striped bass, or barramundi with great results. Any of these white-fleshed fish will take well to the bright flavors of the simple grapefruit-and-orange topping.