Lay out two long pieces of plastic wrap on a rimmed baking sheet. Overlap the two pieces of plastic wrap 2 inches and extend past the baking sheet 6 inches on the long sides (to allow to fully wrap over the salmon).
In a large bowl, combine the salt, brown sugar, coriander, and dill. Sprinkle 1 cup of the salt and sugar mixture on the plastic wrap, making it extend 1 inch past where the salmon will lay.
Place the salmon skin-side down on top of the salt and sugar mixture.
Sprinkle the remaining salt and sugar on top of the salmon, making sure there are no exposed pieces of the salmon remaining.
Lift up the ends plastic wrap and fully cover the salmon, making sure that no ends of the wrap are hanging over the edges of the baking sheet. Place in the refrigerator for 24 hours, flipping the salmon over so it is skin side up after the first 12 hours.
Remove the salmon from the fridge and unwrap. Carefully lift the salmon from the salt and sugar mixture and gently shake off any excess.
Place the salmon in a vessel (such as a 9x13 pan) and fill with cool water until the salmon is covered. Allow the salmon to soak in the fridge for 15-20 minutes. This will draw out some of the salinity and rinse off any particles from the cure.
Place the salmon on a flat, elevated surface that will allow the air to circulate on all sides of the salmon (like a roasting or cooking rack) over a rimmed baking sheet and place in the fridge for at least 4 hours (or up to 8 hours) to develop a tacky coating on the exterior called a pellicle.
Prepare your smoker for cold smoking. Your smoker needs to be under 80 degrees F, so be sure to also check your outside temperature. My smoke set up for this recipe is using a smoke tube and alder or maple pellets in my offset or pellet smoker (with no additional fire running). I have also seen many people use other varieties of cold smoke generators.
Place the salmon directly into the smoker, skin side down. To keep things simple, I just leave it on the rack I used during the pellicle stage. Close the lid and smoke for 18-24 hours, depending on the amount of smoke you want your salmon to take on and how thick your salmon piece is. You are looking for an even light brown color across the top of your salmon and a feeling of firmness (not a lot of give) when you press on the salmon with your fingers.
Remove the salmon from the smoker. Chill completely in the fridge. Serve as desired. I like to slice mine thin against the grain. and serve the bagels, cream cheese, chives, capers, lemon, red onions, etc. This salmon will last for 3 days in the fridge.
The salt and sugar will draw moisture out. This is normal, so if you see salmon juice on your baking sheet, do not be alarmed. The rimmed baking sheet will help contain this while the salmon is in the refrigerator.