Begin by preparing the oxtails. In a large bowl, mix together the oxtails with browning sauce, Worcestershire sauce, kosher salt, and black pepper. Allow the oxtails to marinate for at least 10 minutes.

In a slow cooker with a browning setting or a stovetop, melt 2 tablespoons of butter over medium heat. Add the chopped onions and sauté for about 2 minutes.

Sprinkle all-purpose flour into the sautéed onions and whisk continuously for 3-4 minutes until the mixture is a golden brown.

Slowly pour in the beef broth while whisking the roux to avoid lumps. Add brown sugar, paprika, onion powder, and garlic powder, and mix well.

Carefully place the marinated oxtails into the slow cooker, ensuring they are submerged in the rich gravy mixture.

Cover the slow cooker and set it to low heat for 4 hours. The oxtails should be tender with the gravy thickened and rich.

Once cooking is complete, check the doneness of the oxtails. For thicker gravy, mix 1 tablespoon of cornstarch with cold water and whisk it in.