Preheat your oven to 300°F. Pat the chuck roast dry with paper towels to help achieve a better sear. Generously season all sides with salt and pepper.
Heat the olive oil in a large Dutch oven (6-8 quart size recommended) over medium-high heat. When the oil is hot and shimmering, add the roast. Sear it without moving for 5-7 minutes on each side until a deep golden-brown crust forms. This step locks in the juices and builds flavor. Remove the roast and set it aside on a plate.
Lower the heat to medium-low. Add the diced onion and celery to the pot, scraping up any browned bits from the bottom, which add depth to the dish. Cook for 8-10 minutes, until softened and beginning to caramelize. Stir in the smashed garlic, tomato paste, and soy sauce; cook for 1 more minute until fragrant.
Pour in the chicken stock to deglaze the pot completely, stirring well. Return the seared roast along with any juices that have accumulated to the Dutch oven. Bring the liquid to a gentle boil, then cover tightly with the lid. Transfer the pot to the preheated oven and braise for 2 hours at 300°F. The meat should start becoming tender.
Remove the pot from the oven and add the diced potatoes and carrots, pressing them into the liquid for even cooking. Cover again and return to the oven for another 1 to 1.5 hours, or until the roast is fork-tender and the vegetables are soft but not mushy.
Take the roast and vegetables out onto a serving platter and tent with foil to keep warm. Skim any excess fat from the pot’s liquid if necessary. Place the pot back on the stovetop over medium heat and bring the liquid to a simmer. In a small bowl, whisk the cornstarch with ¼ cup cold water until smooth. Gradually stir this mixture into the simmering liquid and cook for 2-3 minutes until the gravy thickens to your liking. Taste and adjust the seasoning with salt and pepper as needed.
Slice or shred the roast and pour the thickened gravy over the meat and vegetables. Garnish with chopped fresh parsley if desired and serve hot.