Preheat water bath to 185˚F (85˚C).
Cut the butternut squash in half down the center. Cut again to separate the neck and the gourd. Dice the neck and reserve for puree.
Remove the seeds from the gourd and thinly slice on a mandoline.
Place the sliced gourd in a zip top bag. Add the white wine vinegar, water, yellow mustard seeds, sugar, s