Lay salmon fillets skin-side up on a cutting board. Use a sharp knife to cut a few parallel slits through the skin. This step will help get the skin nice and crisp when you sear. It’s optional.
Flip the fillets. Season with salt and brown sugar (if using) generously, using roughly one part sugar for every four parts salt. Let it sit for 10 minutes and rinse with cold water.
Pat the fillets dry with a paper towel and sprinkle with freshly ground black pepper.
Before you bag the fish, add a generous amount of olive oil in a large ziplock bag. The oil will prevent the fish fillets from sticking together after cooking. Alternatively, you don’t need any oil if you use a sous vide bag. Just make sure you leave a little room between the fillets before vacuum sealing.
Place the dill sprigs on the skin side of the fillets and sprinkle with lemon zest. Seal the bag and lower it into the water bath until the salmon is completely under the water. Cook for 40 minutes.
Once the salmon is done, gently transfer it from the bag to a plate, skin-side up.
Heat a cast iron skillet or nonstick pan until it’s very hot, add some oil and flip the fish into the pan, skin-side down. Sear for 30 seconds to 1 minute until the skin is crispy.
Carefully transfer each fillet to a plate and serve.
Before bagging the fish, make sure to use spices of your choice in powder form on your fish and add any fresh herbs on the skin side, so they don’t leave marks on your fish meat after cooking.
Sous vide cook time will vary based on the size of your fish. 1-inch thick filets should cook in about 40 minutes, while half-inch fillets should cook in 30. If your salmon fillets are 1.5-inch thick, leave in the water for an hour.