Uni has long been called the foie gras of the sea, a supple delicacy for advanced palates. At first glance, however, the sea urchin isn’t much to look at. The exterior is reminiscent of those spiky balls that medieval soldiers attached to a chain and used to bludgeon their neighbors. Once it’s cracked open, however, the prize is revealed. The cheddar-hued meat, which looks like a tongue, is actually the gonads. It is quiveringly soft and tastes like, well, a luscious seawater panna cotta. In the United States,…