Mash together the mustard and the butter with a tablespoon of buttermilk; set aside.
Pour enough of the buttermilk to cover the bottom of an au gratin dish. Layer onion slices and potato slices alternately; sprinkle salt and pepper to taste over each potato layer and put blobs of the butter mixture on each layer of potato; ending with a potato layer and butter mixture blobs. Pour the rest of the buttermilk over all. Open layers a little with a fork as you pour to let the buttermilk through.
Sprinkle Swiss cheese shreds evenly over the top. Bake for 45 minutes to 1 hour or until potatoes are fork tender and top is browned and crunchy.