To prepare the pancakes, bring a stockpot of water to a brisk boil. Drop in the potatoes and boil just until they are slightly softened yet still firm, about 10 minutes. Drain and set aside to cool. When cool enough to handle, grate the potatoes. Heat the olive oil on high heat in a 12-inch nonstick saute pan until very hot. Add all of the grated potatoes and press to compact them into one large pancake. Cook the potato pancake for about 8 minutes, or until golden brown. Rub about 1 tablespoon of the butter around the side of the pan. Turn the pancake with a large spatula and brown the other side well, about 8 minutes. Just before the pancake is finished, rub the remaining 1 tablespoon of butter around the edge of the pan. Season the pancake well with salt and black pepper and slide it onto a large serving platter. Mound the relish in the center of the pancake, top with the feta, and a dollop of yogurt or sour cream. Cut into wedges and serve immediately. Per serving: 130 Calories (kcal); 13g Total Fat; (89% calories from fat); 3g Protein; 1g Carbohydrate; 32mg Cholesterol; 210mg Sodium Food Exchanges: 0 Grain(Starch); 1/2 Lean Meat; 0 Vegetable; 0 Fruit; 2 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.