Place the steak flat in a large nonreactive dish such as glass or porcelain. Using the tip of a sharp knife, lightly score the top of the steak diagonally in a large crisscross pattern to make what looks like 1-inch diamonds across the steak. This allows the marinade to penetrate into the meat.
In a 2-cup measuring cup, whisk together the wine, olive oil, mustard, 1 teaspoon salt, and 1/2 teaspoon pepper. Stir in the shallots and garlic. Pour the marinade over and under the steak until it's completely coated. Scatter the tarragon on top. Cover the dish with plastic wrap and allow it to marinate in the refrigerator for at least 2 hours but preferably overnight.
Half an hour before serving, remove the steak from the refrigerator and heat a charcoal grill with a layer of hot coals. (You can also set a gas grill on medium-high heat.) Sprinkle the steak with 1 teaspoon salt and 1 teaspoon pepper. Discard the remaining marinade.
Grill the steak for about 5 minutes per side for medium-rare. Place the steak on a clean plate, cover tightly with aluminum foil, and allow to rest for 10 minutes. Slice the steak thin, cutting diagonally across the grain. Sprinkle with salt and serve hot.