Reduce the stovetop heat to medium
Add the butter to the pot along with the chopped onion, cook, stirring occasionally for 2-3 minutes until softened.
Add in the garlic and cook, stirring constantly for 30 seconds. Then, stir in the tomato paste and cook for one more minute.
Pour the red wine into the pot and use a wooden spoon to scrape up any browned bits from the bottom of the pan.
Pour in the beef stock, Worcestershire sauce, balsamic vinegar, bay leaves, thyme, and return the seared beef to the pot. Stir to combine.
Reduce the heat to low (medium-low), cover and simmer for 60 minutes until the beef ins becoming tender.
After 60 minutes, stir in the carrots, potatoes, and mushrooms.
Cover and simmer for 30 more minutes until vegetables are tender.
If desired for a thicker stew, whisk together the cornstarch and cold water in a small bowl, then whisk into the stew for 1-2 minutes until thick.