Digital kitchen scale, Chef’s knife, Measuring cups, Measuring spoons, Wire cooling rack, 2 half sheet pans, Immersion circulator, Large pot or other vessel for setting up a water bath, Paper towels. 2 gallon-size vacuum-seal or high-quality freezer zip-top bags, Peeler Vacuum sealer (optional), Medium bowl, Rubber spatula, Scissors, Pastry brush, 10-inch cast-iron skillet, Fish spatula Platter
Place the salmon on a wire rack set in a rimmed baking sheet. Rub the fillets with 2 teaspoons of the salt, then refrigerate for 45 minutes. Meanwhile, set your immersion circulator bath for 124 degrees F (51 degrees C).
Pat the salmon dry with paper towels, then divide between two gallon-size vacuum-seal or high-quality freezer zip-top bags. Add 1 1/2 teaspoons of the olive oil to each bag, then gently rub the fish to coat. Use a peeler to harvest eight strips of lemon peel and place two strips, pith-side up, on top of each fillet. Halve and juice the lemon; set the juice aside to use in the sauce. If using vacuum-seal bags, use the “gentle” or “moist” setting to seal the bags.
Transfer the fillet bags to the water bath. If using zip-top bags, let the salmon sink, forcing the air out of the bag via displacement. Right before the mouth of the bag goes into the water, seal the bag. Leave the salmon to cook for 45 minutes. (If one of the bags floats, carefully reopen and try the sink-and-seal method again.)
While the salmon cooks, make the sauce: In a medium bowl, combine the yogurt, dill, mustard, horseradish, and pepper. Stir in the remaining teaspoon of salt and 1 tablespoon of the lemon juice. Stir well, then season to taste with additional salt, pepper, and lemon juice, if desired. Set aside until ready to serve.
When the salmon is finished, remove it from the water bath. Open the bags and very carefully remove the fish to a paper towel-lined sheet pan. Pat the fish as dry as possible with additional paper towels. Brush the skin side lightly with some of the neutral oil.
Heat a 10-inch cast-iron skillet over high heat for 3 minutes. Add 2 teaspoons of the neutral oil and swirl the pan to coat. When the oil starts to smoke, add two of the salmon fillets, skin-side down. Brush the flesh side lightly with additional oil. Cook until the skin is browned and crisp, about 1 minute. Carefully flip with a narrow metal spatula or “fish turner” and cook on the second side just until just brown, about 30 seconds. Carefully transfer to a serving platter and repeat with the remaining oil and fillets. Serve with the dill-yogurt sauce.
To thaw frozen fish, remove the fish from its packaging, wrap each fillet in plastic wrap, and place in a lidded container. Refrigerate overnight and cook the next day. Do not let the thawed fish sit in the refrigerator for more than 24 hours.