Soak prunes in water while preparing other ingredients. Lightly film the bottom of a 12-inch saute pan with oil. Heat over high and add onions, salt and pepper. Saute fast until onions start to color but are still crisp.
Lower heat to medium low, stir in garlic and drained prunes and cook about 3 minutes. Off the heat, blend in capers and horse radish to taste. Sample for seasoning. Serve at room temperature.