Unless you know how fresh they are, always eat mussels on the same day you buy them. Carefully place the blue-black mussels into a sinkful of cold water and discard any that stay open when tapped.

Toasted walnuts are a surprise addition to the creamy, smoky red pepper puree that gets swirled into these harissa-spiked steamed mussels. Recipes published by Food & Wine are rigorously tested by ...