Steamed Black Drum with Mussels

• 4 cups loosely packed, fresh flat-leaf parsley leaves (from about 1 bunch)
• 4 small garlic cloves
• Kosher salt
• 2 pounds mussels, scrubbed
• 12 small waxy potatoes
• 4 (6-ounce) fillets of black drum, skin on 
• vegetable oil
• Freshly ground black pepper
• 3 tablespoons unsalted butter

Chop the parsley. Smash the garlic, choping it just a little to make grinding easier, and then work it into a paste with a small pinch of salt in a mortar. Add the parsley and continue to work the mixture in the mortar until it is smooth.

In a 4-quart pot, bring 2 cups of water to a boil, lightly salt it, and add the mussels. Cover and cook over high heat for 3 to 5 minutes, checking the mussels often and transferring them to a plate as soon as they open. Discard any that do not open. Strain the mussel broth through a fine-mesh sieve and reserve it. Shuck half of the mussels.

Set a large steamer basket over a pot filled with a couple of inches of boiling water, and add the potatoes. Cover and cook until the potatoes are close to halfway done, 4 minutes. Meanwhile, drizzle the fish with oil, season it with salt and pepper, and put it on a plate that will fit in the steamer basket, making sure the fillets do not touch each other.

When the potatoes are about halfway done, add the plate of fish to the steamer basket, and steam for about 6 minutes, until the fish is barely cooked through and the tip of a sharp knife inserted in the thickest part of the fish for 5 seconds comes out warm to the touch. Transfer the fish and potatoes to a warm platter and cover to keep warm.

In a large, deep sauté pan, bring the reserved mussel liquid to a simmer. Add the parsley mixture and cook for 45 seconds. Remove from the heat, add the shucked and unshucked mussels, stir to reheat, and swirl in the butter. Add salt if necessary before spooning the mixture over the fish and potatoes.