This dip is best enjoyed straight out of the oven, so if you're looking for a make-ahead solution, I'd steer clear of making and reheating the entire dish. What you can do, though, is caramelize the onions, stir in the salt and wine, let the wine cook off, and then save the onion mixture in the fridge until you're ready to make and serve the dip. When it's time for that, simply spread the onions in the bottom of a skillet, top with cheese, and then bake until warmed through.