Melt the spread in a small skillet. Add the onion and apples and sauté until the onion is translucent.
Whisk together the vinegar, mustard seed, and sugar in the bottom of a 4-quart slow cooker until the sugar dissolves. Add the remaining ingredients. Stir to evenly distribute. Cook on low for 8 hours or on high for 4 hours. Serve hot.