Preheat the oven to 375º.
Heat a large cast-iron skillet over medium heat. Add 1 tablespoon of oil to the skillet. When the oil is screaming hot, sear the skin-side of each fillet for 45-60 seconds, or just until the skin is crisp and starting to brown. Remove the fillets and place skin-side up on a plate.
Add another tablespoon of olive oil to the skillet and then the leeks. Season with a few pinches of sea salt. Sauté the leeks until golden brown, stirring frequently, about five to eight minutes. Remove from the skillet and place into a small bowl.
Cover the bottom of the skillet in an even layer of lemon rounds. It's fine if they overlap slightly. Drizzle with the remaining tablespoon of olive oil. Let them cook until they begin to turn golden brown, then sprinkle them with the leeks and add the salmon in, crispy skin-side up (cut-side of the salmon down, touching the leeks and lemon). Turn off the stovetop and move the skillet to the preheated oven for 7-10 minutes, until the fish is just cooked through.
Remove from heat, sprinkle with flaky sea salt and pepper, and top with salsa verde and sliced radishes. Serve with extra lemon.