1tablespooncornstarch (or ½ teaspoon konjac flour)
Place the oxtails in your slow cooker.
In a medium bowl, whisk together the remaining ingredients except for the cornstarch. Pour over the oxtails, and stir to coat.
Cook on low for 8 hours, until the meat is very tender.
Temporarily transfer the cooked oxtails to a platter. Strain the liquids from the slow cooker into a saucepan. Return the oxtails to the slow cooker, cover with the lid, and set to WARM.
Heat the cooking liquids on medium heat. Whisk the cornstarch with a tablespoon of cold water and stir it into the saucepan. Keep cooking the sauce, whisking often, until it has thickened into a rich glaze, about 5 minutes.
Remove the oxtails from the slow cooker. Pour the sauce over the oxtails and serve.