Is there anyway to make these ahead of time and then reheat them?

Yes, I've done it, actually! I reheated the potatoes in a sauce pan on the stovetop, stirring frequently and adding milk and butter to keep them from being too dry. I rewarmed the caramelized onions separately in a saute pan with a little butter. It worked out just fine!

You can definitely make mashed potatoes ahead of time and keep them warm over a simmering pot of water. If you're talking a day or so then I would reheat them the same way.