Put the vegetable oil in an iron cast pot large enough to hold the ingredients, preferably with a heavy bottom. Heat the oil over medium-high heat. Add the beef chunks and sauté for a few minutes.

When the color of the beef changes, add the onions and garlic. Sauté for a few minutes until the onions become translucent and the garlic is fragrant.

Add the broth, water, mushroom gravy, and brown sugar to the pot, stir occasionally, and bring the mixture to a boil.

In the meantime, prepare the thickening roux. In a small bowl, whisk cornstarch with water until there are no lumps. Add the roux to the pot and stir to incorporate it into the mixture. Cover the lid, and on low heat, cook the beef mixture for two hours.

After the meat has cooked for two hours, then add the potatoes, carrots, thyme, black pepper, and salt to the pot. If it is too thick, add water. Cover the lid and cook for another hour on low heat.

Check the beef for doneness. With a spoon or spatula, press onto the middle of a piece of beef. If it breaks, it's tender enough. If not, you can cook for an additional 10-15 minutes.

When the beef becomes spoon-tender, turn off the stove, and with the lid closed, leave the stew to rest for half an hour before serving.