In a bowl, mix kosher salt, brown sugar, lemon zest, orange zest, dill, thyme, and crushed peppercorns.
Lay out a large piece of plastic wrap. Spread half of the cure mixture on it, slightly larger than your salmon fillet.
Place the salmon, skin-side down, on the cure mixture. Cover the flesh side with the remaining cure.
Wrap the salmon tightly in plastic wrap, then place in a shallow dish.
Refrigerate for 24-48 hours, flipping once halfway through.
After curing, rinse the salmon thoroughly under cold water and pat dry with paper towels.
Place the salmon on a wire rack and refrigerate, uncovered, for 2-4 hours to form a pellicle (a tacky layer that helps smoke adhere).
Prepare the smoker for cold smoking, keeping the temperature below 80°F.
Smoke the salmon for 8-12 hours, or until it reaches your desired level of smokiness.
Once smoked, wrap the salmon in plastic wrap and refrigerate for at least 4 hours or overnight before slicing.