Heat the vegetable oil for frying to 375F in a deep skillet or deep fryer. In a small bowl, stir together the flour, cornstarch, and pepper. Mix the sparkling wine into the dry ingredients until they are thoroughly combined into a thick, smooth batter. Shuck the oysters by carefully inserting an oyster knife into the oyster near the hinge and firmly twist it to open the shell. Gently scrape the oyster meat loose and transfer it to a colander set over a plate or sink to drain. Repeat with the remaining oysters. Dip the oysters into the prepared batter and fry them in batches of 4 in the hot oil, turning once halfway through, until they turn deep, golden brown, about 4 to 5 minutes. While the oysters are frying, toss the horseradish, lemon juice, and lemon zest in a small bowl and set aside. Serve the oyster fritters hot, drizzled with the red wine vinegar or the horseradish-lemon garnish. This recipe makes 8 appetizer-sized servings or 4 main course-sized servings.