In a large skillet, heat oil over medium. Add onion, apples, and raisins and cook, stirring occasionally, until onion is translucent, 4 minutes. Add ginger and coriander and cook until fragrant, 1 minute. Add brown sugar, vinegar, peppercorns, salt, and chiles (if using); bring to a simmer and cook, stirring occasionally, until fruit is tender and liquid thickens, 20 minutes. Serve warm or at room temperature.