Heat a large, well-seasonedcast iron skillet over medium-high heat. A well-seasoned skillet is important to prevent sticking.
Place the salmon skin side up on a plate. Pat the skin dry with a paper towel, then brush with a thin layer of oil. Sprinkle generously with salt and pepper.
When the pan is hot (test it by sprinkling some water in — it should sizzle), gently place the salmon in the pan skin side down and press it down so all of the skin makes contact with the pan (otherwise the skin will curl up). Reduce the heat to medium.
Sear undisturbed for 6-7 minutes*, until the skin is crisp and the fish is cooked ~1/2 - 3/4 of the way up the sides of the filets (you can tell by the color — it will turn lighter pink as it cooks). Brush the top of the salmon with oil and sprinkle with salt and pepper.
Use a spatula (and tongs if necessary) to gently flip the filets. Add the juice of 1/2 lemon to the skillet and cover. Cook for 1-2 minutes, or until the internal temperature reaches 145 F (63 C // temperature recommended for food safety). Depending on the thickness of the fish, it can take up to 4 minutes on the second side.
*Cook time depends on the thickness of your filets. Super thick (1.5+ inches) will take up to 10 minutes on the first side and 3-4 on the second. *The reason to brush the fish with oil instead of adding it to the pan is to prevent smoking — as the pan heats up, all of that oil will hit its smoke point (which is why it often smells fishy the next day if you don’t have a good vent). *A ~4-ounce raw filet will cook down to ~3 ounces. *Nutrition information is a rough estimate calculated with 4-ounce filets, avocado oil, and 1/8 tsp each of sea salt and black pepper.