Preheat the oven to 400 degrees F.
Lightly grease 2 large baking sheets with olive oil (about 1 teaspoon each).
Cut the potatoes in 1/2 lengthwise. In a bowl, toss with the remaining oil to coat, and lightly season with salt and pepper. Place, cut sides down, on the baking sheets and roast until just tender, 20 to 22 minutes. Remove from the oven and let sit until cool enough to handle. Leave the oven on.
Meanwhile, preheat a grill or grill pan over medium-high heat. In a small bowl, make a paste with the vegetable oil and Essence. Rub the meat on both sides with the paste. Place the steak on the hot grill (or on the grill pan) and cook until medium-rare, about 8 minutes per side. Remove from the grill and let rest for 10 minutes.
With a small melon baller or small spoon, carefully scoop the pulp from each potato, leaving a thin shell. Place the pulp in a saucepan, and return the potato shells to the baking sheets. To the saucepan with the potato pulp, add 1/2 cup of the sour cream, the butter, horseradish, salt, and pepper, and mash over low heat with a potato masher until creamy, adding more sour cream as needed to keep moist. Remove from the heat and add the green onions and parsley, and stir well. Adjust the seasoning, to taste. Cover to keep warm.
Return the potato shells to the oven to warm through, about 3 to 4 minutes.
To assemble, slice the steak thinly and cut into 50 pieces, each about 2 1/4-inches long. Divide the mashed potato filling among the potato shells, mounding slightly. Fold each steak strip into 1/3's and arrange on top of each stuffed potato. Place on a decorative platter, sprinkle with chives and serve.
Yield: 50 bites