Heat the oil in a skillet over medium-high heat. Season the roast with salt and pepper before adding it to the skillet to brown on all sides. Transfer the roast to your slow cooker; drain the excess fat from the skillet and set back on the heat. Place the stock, flour and mustard in a bowl and whisk to combine. Pour into the skillet, add all remaining ingredients, and bring to a boil. Once it boils, remove from the heat and pour the mixture over the roast. Cover and cook the pot roast in the slow cooker (on the low setting) for 10 hours. Slice the pot roast and serve the cooking liquid alongside for spooning on top.
Note: This recipe is designed for an oval-shaped slow cooker with a 5 litre or more capacity.