Step 1: Make the Peppercorn Cream Sauce: In a heavy saucepan over medium heat, melt the butter. Stir in the shallots and sauté until soft. Add the black pepper and Cognac. If desired, carefully flambé the warm Cognac with a long match. Otherwise, continue with the sauce. Add the white wine and bring to a boil; reduce by half. Stir in the cream and mustard and let the sauce simmer until it coats the back of a spoon. Add salt to taste. Stir in the chopped fresh tarragon and squeeze in a few drops of fresh lime juice. Keep warm until ready to serve.

Step 2: Working on a rimmed baking sheet or baking dish, season the tuna loins well on all sides with salt and pepper. Drizzle with olive oil.

Step 3: For each loin, cut 4 lengths of butcher’s string, each about 12 inches long, and lay them parallel to each other (about 1-1/2 to 2 inches apart) on a clean work surface. Place a strip of bacon in the center, perpendicular to the strings. Lay a tuna loin on the bacon. Lay 2 more strips of bacon on top of the tuna (3 if it’s a large loin), bring the strings snugly over the loin, and tie so that the bacon is held firmly in place. Repeat with the other loin.

Step 4: Set up your grill for direct grilling and preheat to medium-high (400 degrees). If using a plancha or griddle, preheat it at the same time. Brush and oil the grill grate and/or plancha.

Step 5: Arrange the tuna loins on the plancha or grill grate and grill until the bacon and tuna are seared, 2 to 3 minutes. Turn with tongs to sear the remaining sides. The interior of the tuna should retain its rosy color; do not overcook. Transfer the tuna loins to a cutting board. Reheat the Peppercorn Cream Sauce, if necessary. Remove the strings from the tuna, then slice the loins into 1-inch-thick medallions and arrange on a platter. Spoon the sauce around the bottom of the medallions, garnish with fresh tarragon sprigs, and serve.