In a large skillet, toast the pecans over medium heat until fragrant. Remove from the pan and set aside.
In the same skillet, melt the butter over medium heat. Add the leeks, season with 1 teaspoon salt and cook, stirring frequently, until soft, 8 to 10 minutes. Add the vermouth and cook until reduced by two-thirds. Stir in the cream and mustard and cook, stirring, until heated through. Lay the scallops in the pan and cook for 1 minute, then flip over and cook until they just turn white, 2 to 3 minutes longer. Season with pepper and sprinkle with the toasted pecans.