In a jar or small bowl with a fitted lid, combine the Dijon mustard, sherry vinegar and olive oil with the garlic cloves. Add a pinch of salt. Cover and shake vigorously to blend. Taste for seasoning. Set aside.
Fill a medium bowl halfway with ice cubes and add some cold water. Place a colander squarely inside the ice bath. The colander will keep you from having to pick the green beans out from amongst the ice cubes. Bring a medium pot of water to a boil. Add salt until the water tastes like seawater. Add a generous pinch of sugar. Add ALL BUT A FEW OF the green beans and cook for 2 minutes. Remove them from the water with a strainer and transfer them to the colander inside the ice bath. Allow them to sit in the ice water for a couple of minutes to assure they have cooled thoroughly. Discard the blanching water.
Remove the beans from the ice bath and spread them out on a kitchen towel on a flat surface. Use another kitchen towel to gently pat them dry. This step will prevent the water from diluting the flavor of the beans and the vinaigrette. Transfer to a medium bowl and season with salt. Stir to blend. Remove the garlic cloves from the vinaigrette and toss with the beans, basil leaves and olives. Cut the reserved raw beans into 1-inch pieces and toss with the other beans.
Heat a large cast-iron skillet. Add the oil. When it begins to smoke lightly, sprinkle the steak with salt and use a pair of metal tongs to gingerly place the steak in the hot oil. Cook over high heat 3 to 4 minutes on each side. I count 8 to 10 minutes for medium-rare, depending on thickness. If you like your steak a little more cooked, leave it in the pan a few minutes longer on each side. Remove the steak and allow it to rest on a flat surface, 10 minutes. Slice the steak and arrange on a platter with the green bean mixture.