Season the chuck roast generously on all sides with salt and pepper. Place the chuck roast in a large roasting dish then pour in the beef stock, onion, garlic, tomatoes and spices.
Cover with foil then place in the oven and allow to cook for 2-3 hours until the beef is soft and tender.
Remove the beef from the roasting pan then strain the cooking liquid into a measuring jug.
Pour 3 cups of cooking liquid into a saucepan and add a few teaspoons of cornstarch mixed with two tablespoons of water.
Allow to simmer over medium heat until thickened.
Shred the beef (I find it easiest to put the beef in the bowl of a stand mixer fitted with a paddle attachment to quickly shred the beef).
Combine with 1 cup of the thickened cooking sauce then season to taste.
Fill each tortilla with the shredded beef mixture and a bit of shredded cheese then roll up and place in a baking dish.
Pour the remaining sauce over the top then add the remaining cheese.
Place the enchiladas back in the oven and allow to bake for 10-15 minutes until hot and bubbling.
Remove from the oven and allow to cool for 5 minutes then top with avocado, fresh cilantro leaves and sour cream.