Combine the garlic and parsley in a food processor and puree until smooth. With the motor running, pour the oil through the feed tube in a steady stream. Add the salt and cayenne and pulse a couple of times to mix well.
Combine the potatoes, onions, salt, and black pepper in a salad bowl. Add the garlic dressing and toss to coat thoroughly.
Let sit at room temperature or chill for 30 minutes before serving.
Yield: 6 servings