Recipe adapted from Thank You for Smoking by Paula Disbrowe (Ten Speed Press, 2019).
For the pickled mustard seeds:
¾ cup yellow mustard seeds
1 cup distilled white vinegar
⅓ cup sugar
1 dried arbol chile
½ teaspoon kosher salt
For the salmon:
1 (4-pound) skin-on whole side of salmon, pin bones removed
3 tablespoons olive oil, plus more for drizzling
Kosher salt to taste
1 tablespoon Voatsiperifery peppercorns, plus more to taste
5 or 6 sprigs rosemary, thyme, or dill
3 handfuls watercress, cleaned and trimmed
1 handful sunflower sprouts
3 tablespoons pickled mustard seeds
2 tablespoons white wine vinegar
Flaky salt for garnish
Lemon wedges for serving
Make the pickled mustard seeds: Rinse mustard seeds in a fine mesh strainer, pat dry with paper towels, and place in an 8-ounce glass jar or heatproof bowl. In a small saucepan, combine vinegar, sugar, chile, and salt. Bring mixture to a boil over medium heat, stirring to dissolve the sugar. Pour hot liquid over mustard seeds and let soak at room temperature until softened, about 3 to 4 hours. Store pickled mustard seeds in a sealed jar in refrigerator for up to 4 months.
Make the salmon: Measure out two sheets of heavy-duty aluminum foil that are 12 inches longer than the salmon and place them on a rimless baking sheet. Place salmon skin-side down in center of foil, drizzle with enough olive oil to lightly coat, and generously season with salt, pepper, and herbs, using your fingers to spread evenly over fish. Set salmon aside to marinate at room temperature while you prepare the grill.
Prepare a charcoal grill for two-zone cooking and build a medium-high fire, or heat a gas grill to medium-high. When coals are glowing red and covered with a fine gray ash, use tongs to remove cooking grate and place a drip pan with 1 inch of warm water on the side with no coals. Add your smoke source (chips, chunks, or a log). Return cooking grate, allow to preheat, then carefully wipe with a lightly oiled paper towel. Scrape grate clean with a grill brush, then wipe with oiled towel again.
When the fire begins to produce a steady stream of smoke, carefully slide the foil and fish off the baking sheet and onto the grill over indirect heat. Close the grill, vent for smoking, and smoke for 25 to 30 minutes. Be sure to rotate the fish as needed to ensure even cooking. When salmon is cooked (it will feel just firm and flake easily with a fork), carefully slide foil and salmon back onto baking sheet and let rest for 10 minutes.
Meanwhile, in a mixing bowl, combine watercress and sunflower sprouts. Add mustard seeds, white wine vinegar, and 3 tablespoons olive oil and toss to combine. Season with salt and pepper to taste. When salmon is ready, garnish with a light sprinkle of flaky salt and serve alongside watercress salad and lemon wedges. Refrigerate leftover salmon in a sealed container for up to 4 days.