Add onion to the bottom of a baking pan. Top with boneless flanken. Coat with coffee rub–you will not need to use all of it; just cover generously. You can shmear first with a little oil to help it stick if you like.
Add wine and water to the pan.
Cover and bake for 3-4 hours (or more, if your oven is packed when cooking for Yom Tov), until meat is tender.