In a sauté pan coated lightly with oil, sauté the onions and lemongrass over high heat until soft, about 5 minutes. Season with salt and pepper. Add the cranberries and sugar and deglaze with the rice vinegar. Reduce by 75 percent, or until the liquid is absorbed. Check again for seasoning. When cool, transfer to a container, cover, and store in fridge for up to 2 weeks.