1/4 cup chopped thyme leaves, plus thyme sprigs for garnish
1 teaspoon unsalted butter
Preheat the oven to 350°. Season the lamb with salt and pepper. Heat the olive oil in a large skillet. Add 2 lamb loins and brown on all sides over high heat. Remove the loins to a platter and repeat with the remaining lamb. Let cool.
Pour off the fat from the skillet. Add the wine and water and simmer over moderate heat for 5 minutes, scraping up the browned bits from the bottom of the pan. Strain this lamb jus into a small saucepan.
Brush the lamb loins with the mustard and arrange on a baking sheet. Sprinkle with the chopped thyme leaves and roast in the oven for 15 to 20 minutes for medium rare. Transfer the lamb to a cutting board.
Bring the reserved lamb jus to a simmer over moderate heat and cook until it has reduced by one-fourth, about 4 minutes. Whisk in the butter and season with salt and pepper.
Slice the lamb crosswise into 1/2-inch medallions. Drizzle with the jus, garnish with the thyme sprigs and serve.
A concentrated Merlot, with its palate-cleansing tannins, is a bright contrast to the lamb.