Black-shelled mussels are in their season now, one of the better — and better-priced — sources of protein for quick meals. Buy mussels at markets or grocers that traffic in fish; turnover is key to freshness. That means specialty grocers and many Asian markets. While you may find them at larger retailers, you may safely thumbs-up or thumbs-down with a simple sniff of them: they should have no smell at all or merely a whiff of a sea breeze.