In a large skillet, heat the oil. Saute the leeks and garlic for 2 minutes, stirring frequently. Add the mushrooms, oregano, salt, and black pepper. Reduce the heat slightly and continue to cook, stirring occasionally, until the mushrooms are cooked and their liquid has evaporated, about 5 to 10 minutes. Puree in a blender or food processor and stir in lemon juice and horseradish to taste. Variation: Before pureeing, this combination makes a fine filling for turnovers; if necessary, you can stretch the amount by blending in some mashed tofu and additional seasonings. Makes 1- quarter cups