2 tablespoons kosher salt, plus more as needed
1 tablespoon sugar
2 cups water
4 skin-on mahi-mahi fillets (5 to 6 ounces each)
1 teaspoon Dijon-style mustard
1 teaspoon sugar
Juice of 1/2 lemon
1/2 cup low-fat buttermilk
About 15 mint leaves, chopped (3 tablespoons)
8 nectarines (may substitute white peaches)
1 tablespoon olive oil
1 pound watercress stemmed, rinsed and dried
Freshly ground black pepper
Combine the 2 tablespoons of salt, sugar and water in a baking dish to form a brine, stirring until the salt and sugar have dissolved. Place the fillets in the dish; allow to sit for 20 to 40 minutes.
Heat an ungreased ridged cast-iron grill pan over medium heat, or prepare an outdoor grill for direct and indirect heat. If using a gas grill, preheat to high for 10 minutes with the lid closed. If using a charcoal grill, light the charcoal or wood briquettes; when the briquettes are ready, distribute them on one side of the cooking area. For a medium-hot fire, you should be able to hold your hand about 6 inches above the coals for about 4 or 5 seconds. Have ready a spray water bottle for taming any flames. Brush the grill grate.
Meanwhile, whisk together the mustard, sugar, lemon juice and buttermilk in a liquid measuring cup. Add the mint; let the mixture sit for at least 20 minutes and up to 2 hours (the mint will begin to lose its flavor after that).
Cut the nectarines in half and remove the pits. Brush the cut sides generously with olive oil and season liberally with salt.
Discard the brine; use paper towels to pat the fish dry. Lightly brush the fillets with oil and season lightly with salt.
Place the fillets skin side up on the direct-heat side of the grate. Grill for about 3 minutes (for a 1-inch-thick fillet), then move the fish to the indirect-heat side of the grill, turning the fish over so the skin side is down. Place the nectarines cut sides down on the direct-heat side of the grate. The fish will cook on the indirect side of the heat for 5 minutes, about the same total time it takes to grill the nectarines.
Close the grill cover and grill vents; this will create a smoky flavor in the fish. Grill for about 8 minutes, turning the nectarines over halfway through. If they begin to get too burned, move the fruit to the indirect-heat side of the grate.
While the fish and fruit are cooking, place the watercress in a mixing bowl. Add half of the buttermilk-mint dressing and toss to combine, then divide the dressed greens among individual plates.
When the fish is done, place 2 warm nectarine halves in the center of each salad portion.
Carefully remove and discard the skin from each fillet, then place the flesh of each fillet on top of each salad portion.
Divide the remaining nectarine halves evenly among the portions, placing them on top of the fish. Spoon the remaining dressing over the salad and drizzle it around the plate.
Garnish the salad with a few grinds of black pepper. Serve immediately.