Miniature Servings of Mussels with Sea Urchin Sauce

People who aren’t in the habit of going to sushi bars get spooked when I mention sea urchins, which they often confuse with sea anemones, those ghastly-looking tentacled creatures we’ve all seen at the aquarium. While it is true that sea urchins aren’t particularly beautiful—they’re black, round, spiked, with a hole in the middle—and are especially nasty if you step on one at the beach, they have their aficionados. In France, you’ll find sea urchins served along with oysters and other raw s