Preheat oven to 300 degrees f.

Peel and cut onion into half inch chunks. Peel and cut carrots and parsnips into one inch chunks. Mince garlic. If the potatoes are larger than an 1.5 inches, halve them.

Season the chuck roast all over with salt and pepper.

Heat oil over medium high heat in a large dutch oven. Place roast in the hot pan and sear on one side for 3-4 minutes without moving, allowing a crust to form on the meat. Flip and sear on each side for another 2-3 minutes for a total of about ten minutes.

Using tongs, remove the roast from the pot and set aside on a plate.

Turn the heat down to medium low and add the onions and leeks to the pot. Sauté for 2-3 minutes, just until softened. Add in the minced garlic and cook for another minute until fragrant.

Add in the red wine to deglaze the pan and use a wooden spoon to scrape up any browned bits on the bottom of the pot. Cook for another minute or two.

Stir in beef broth, dijon mustard, Worcestershire sauce and bay leaf and bring to a simmer.

Add the roast back to the pot, making sure the liquid comes mostly up the sides of the roast but not covering it. If you need a bit more liquid, just add in more beef broth.

Put the lid on the dutch oven and bake for 1.5 hours.

Add potatoes, mushrooms, carrots and parsnips to the pot, making sure they are nestled down in the liquid and not on top of the roast. Bake for another 2 hours or until the veggies are tender and the meat falls apart when gently pulled with a fork. If the meat is still tough, continue to bake until tender.

Use tongs to transfer the roast to a serving platter and a slotted spoon for the veggies. Garnish with fresh parsley and cover with foil to keep warm while making the gravy.

Skim as much fat as possible off the top of the pan juices or use a fat separator. Place pot back on the stove and bring the liquid to a simmer.

In a small bowl, dissolve cornstarch in cold water to make a slurry. Stir it into the pan juices a little bit at a time until you reach desired thickness.