Season the scallops with salt and pepper. Heat the oil in a large skillet set over medium-high heat. When the pan is hot, cook and sear the scallops 1 minute on each side, remove from the skillet and set on a plate. Reduce the heat under the skillet to medium, and then add the leeks and garlic and cook until the leeks soften, about 3 to 4 minutes. Add the lemon zest and lemon juice and simmer until the lemon juice has reduced by half. Add the cream and tarragon and bring to a simmer. Simmer and reduce the cream until lightly thickened. Season with salt and pepper. Return the scallops to the skillet, heat through a minute or two, and then serve.