Mussels are tricky. Overwhelm them with strong flavors and you strangle their delicacy. Leave them alone — that is, simply steam them open with a little chicken stock or white wine — and they can seem too slack and fleshy, with a softly funky smell like a tidepool left to evaporate once the surf recedes.
The cooks at Nopa walk a chalk line when it comes to the little bivalves, often tossing them with a surprisingly assertive flavoring: the Tunisian hot sauce harissa. It's house-made, says Nopa's Laurence Jossel, a blend of New Mexican and flaked red chiles, garlic, cumin, coriander, olive oil, and a measure of tomato the chef calls "a smidge." The presence of harissa makes the sauce at the bottom of the bowl potentially incendiary, but here it blends with the mussels' juices, a squeeze of lime, and enough butter to emulsify into a quick pan sauce so smooth it seems enriched with cream, but more fluid, silkier than that. The muffled bite of chile, the scorched-straw taste of cumin: They make the mussels seem unbelievably sweet, though it helps that Jossel sources smaller, sweeter West Coast specimens, usually Tomales Bays or Penn Coves. Turns out harissa is the gentle shock they need to come alive.
A slab of toasted Acme Italian bread — smoky from the wood grill — turns to custard as it sits, absorbing the brick-orange sauce. You slice off hunks with your spoon after the mussels are no more than a pile of spent shells in the boneyard, the smell of a particularly sun-bleached sea on your fingers.