Rinse the scallops and pat them dry with paper towels. Season them with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper. Heat 1 tablespoon of the oil in a large cast-iron skillet over medium-high heat. Add the scallops and cook until golden brown and cooked through, 2 to 3 minutes per side. Transfer to a plate.
Wipe out skillet and return to medium-low heat. Add the remaining oil and the leeks and toss. Cover and cook, turning occasionally, until softened, about 10 minutes. Season with the remaining salt and pepper. Return the scallops to skillet to rewarm. If using, squeeze on the lemon juice or spoon the pesto over the top. Transfer to individual plates.