In a small bowl, combine the Kosher Salt, Black Pepper, White Pepper, and Italian Seasoning and set it aside. This is going to be your Oxtails Seasoning Blend.
In a small bowl, combine the browning and Worcestershire sauce, stirring to comibne. Set aside.
Place the oxtails in a large mixing bowl and pour the Browning and Worcestershire sauce mix over the top.
Using your hands, tongs, or a large spoon, move the oxtails around to ensure they are fully covered.
Remove the oxtails to a plate and season all sides generously with the seasoning mix.
Heat the vegetable oil in a cast iron skillet over medium-high heat.
Sear all sides of the oxtails in the skillet.
Once the oxtails are all browned up, place them inside the slow cooker.
Leaving all the remaining oil in the skillet, add ½ cup of flour and immediately whisk.
Once the oil is all mixed into the flour begin slowly pouring in the beef stock and continuously whisking. Continue this process until you have incorporated all the flour a total of 3 cups of beef stock whisking until there are no clumps.
Lower the heat to medium and stir in the garlic.
Once the garlic is fully mixed in, add the bell peppers and onions.
Stir the mixture to ensure all vegetables are covered in the gravy mixture.
Pour one cup of beef stock into the slow cooker.
Pour the gravy, onion, and bell pepper mixture over the oxtails, spreading it all out to ensure even distribution.
Place the lid on the slow cooker, set to high, and cook for 8 hours.