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1
Preheat oven to 325u00b0F (163u00b0C). Dry salmon very well with paper towels, then season all over with salt and pepper. In a large cast iron, carbon steel, or stainless steel skillet, heat 2 tablespoons (30ml) oil over medium-high heat until shimmering. Reduce heat to medium-low, then add a salmon fillet, skin side down. Press firmly in place for 10 seconds, using the back of a flexible fish spatula, to prevent skin from buckling. Add remaining fillets one at a time, pressing each with spatula for 10 seconds, until all fillets are in the pan.
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2
Cook, pressing gently on back of fillets occasionally to ensure skin makes good contact with pan, until skin releases easily from pan and is crispy, about 6 minutes. Transfer salmon, skin side up, to a platter and set aside.
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3
Add remaining 2 tablespoons (30ml) oil to skillet and heat over medium-high heat until shimmering. Add leeks and cook, stirring, until they begin to soften and compress (tongs can help move them around more efficiently), about 3 minutes. Add curry powder and stir well to combine. Continue cooking until leeks are very tender, about 6 to 8 minutes longer. Lower heat at any point to prevent scorching. Season with salt and pepper (be careful, as some curry powders contain salt).
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4
Arrange salmon on top of leeks, skin side up, and transfer to oven. Cook salmon until an instant-read thermometer registers 110u00b0F (43u00b0C) in the very center for rare, 120u00b0F (49u00b0C) for medium-rare, or 130u00b0F (54u00b0C) for medium, 5 to 7 minutes.
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5
Meanwhile, in a medium bowl, whisk together yogurt, cucumber, garlic, and herbs. Season with salt and pepper, then add lemon juice, 1 teaspoon at a time, to taste. (Yogurt sauce can also be made in advance and refrigerated up to 5 hours before serving.)
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6
Transfer salmon and curried leeks to plates and spoon cold yogurt sauce alongside. Serve.
