Dry the meat with paper towels, then season generously with salt and pepper.
Heat the olive oil in a large frying pan and brown the meat on all sides. Set aside on a plate.
Deglaze the pan with a splash of wine and pour the pan juices into a container to use later.
Mix the corn starch with the cold water to make a slurry. Mix the beef stock with the reserved pan juices, soy sauce, Worcestershire sauce, corn starch and English mustard.
Add a little more oil to the pan and pan fry the onions and garlic over medium-low heat. Season and sprinkle them with the sugar and continue to cook, stirring for 7-10 minutes.
Add the chopped mushrooms and pan fry for a minute then add the stock. Bring to a simmer and transfer to your slow cooker. Add the bay leaf and thyme sprig. Add the brisket, fat side up, and cook for 4 hours on high or 7 hours on low.
Remove the brisket from the slow cooker and put on a plate, loosely covered with foil.
Add the cream to the slow cooker and cook on high, uncovered, for another 20 minutes. Stir in the chopped parsley.
Slice the brisket and serve with mashed potatoes and smothered in the mushroom gravy.