1 small yellow onion, thinly sliced
1/4 teaspoon salt ( or to taste)
3/4 cup water
1 teaspoon Dijon mustard
1 lb Brussels sprout, trimmed, halved and very thinly sliced
1 tablespoon cider vinegar
Add onion and salt to the drippings in the pan and cook over medium heat, stirring often, until tender and browned (about 3 minutes).
Add water and mustard, scraping up any browned bits, then add Brussels sprouts and cook, stirring often, until tender (4 to 6 minutes).