Heat the oil in a large pan over medium high heat. Season the brisket on both sides with salt and pepper.
Place the brisket in the pan. Cook for 6-8 minutes on each side or until browned.
Coat a large oval slow cooker with cooking spray.
Place the brisket in the slow cooker along with the onion, carrot, celery, Italian seasoning, garlic, bay leaf, beef broth and ketchup. Stir to combine.
Cover and cook on low for 8 hours.
Remove the meat from the crockpot. Strain the liquid into a saucepan.
Place the pan on the stove over high heat and bring to a simmer. Cook for 5 minutes.
Place 1/4 cup cold water in a small bowl. Whisk in the cornstarch.
Add the cornstarch to the saucepan. Cook for 1-2 minutes or until sauce has thickened.
Shred the brisket or slice it against the grain. Pour the sauce over the top. Sprinkle with parsley, then serve.