Step 1Pat the salmon dry. In a small bowl, combine the kosher salt with the sugar and 5 grams lemon zest. Rub the mixture over the meat side of the salmon fillet. Vacuum seal the fillet in a bag at maximum pressure. Refrigerate the salmon for 24 hours. (If a vacuum sealer is not available, wrap the salmon tightly in plastic wrap and refrigerate for 24 hours, skin-side down.)
Step 2After curing the fish for 24 hours, remove the salmon from the bag (or plastic wrap), gently rinse and dry very well. Be sure to "squeegee" as much moisture from the skin as possible using a flat-sided tool, such as the back of a knife or a metal spatula, lightly rubbing it repeatedly over the skin-side of the fish and wiping it dry.
Step 3Cut the fillet into 4 equal portions. Place the salmon, skin side up, on a plate and set aside at room temperature for the fish to temper before sauteing.
Step 4Heat a large saute pan over high heat. Add the canola oil so it reaches a depth of about one-sixteenth of an inch in the pan.
Step 5When the oil is hot, place the salmon fillets, skin side down, into the oil. Cook until the skin begins to crisp, about 30 seconds, then reduce the heat to medium-low and continue to cook until about one-third of each fillet is cooked through.
Step 6Flip the fish so it is flesh side down in the pan, add the butter and reduce the heat to low. Continue to cook until another third of each fillet is cooked through. Remove from heat.
Step 7Remove the salmon from the pan and set aside to rest, skin side up. Keep the fish warm until ready to plate, then garnish with the Maldon salt and remaining lemon zest.
Step 1Toast the hazelnuts: Heat the oven to 350 degrees. Place the hazelnuts on a rimmed baking sheet and bake until golden brown and fragrant, about 10 minutes. Remove from the oven and cool. Coarsely crack the hazelnuts with a mallet or the bottom of a small skillet and dress with a drizzle of hazelnut oil and kosher salt. Set aside in a warm place.
Step 2Place a salmon fillet in the center of each plate. Drizzle a little of the lemon hazelnut vinaigrette around it. Off to the side, arrange a few asparagus spears and lean the beets against the fish. Add a fourth of the roasted hazelnuts to each and some of the watercress mousseline. Serve immediately.