Preheat the oven to 325 °F and set a rack in the lower-middle position. Pat the beef dry and season with 2 teaspoons kosher salt and 1 teaspoon ground black pepper. In a large Dutch oven or oven-safe heavy soup pot, heat 3-4 tablespoons of the oil over medium-high heat until hot and shimmering.
Brown the meat in batches, turning with tongs, for about 5 minutes per batch; add more oil, if needed. (To sear the meat properly, do not crowd the pan and let the meat develop a nice brown crust before turning with tongs.) Transfer the beef to a large plate and set it aside.
Add 2 more tablespoons of oil to the pot and reduce the heat to medium. Add the mushrooms and cook, occasionally stirring, until well browned, about 2- 3 minutes. Add the roughly chopped onion and poblano pepper and cook, occasionally stirring with a wooden spoon and scraping up the browned bits on the bottom of the pot, until the vegetables soften, about 5 minutes. Add the balsamic vinegar and cook, stirring, for about a minute.
Move the vegetables to one side and add the tomato paste to the empty side. Cook until it darkens around the edge, about 2 minutes; stir into the vegetables. Add the beef with its juices back to the pan and sprinkle with the flour. Stir with a wooden spoon until the flour is dissolved, about 1 to 2 minutes.
Add the wine, water, bay leaf, thyme, sugar, and beef flavor bouillon. Stir with a wooden spoon to loosen any brown bits from the bottom of the pan and bring to a boil. Cover the pot with a lid, transfer to the preheated oven, and braise for 2 hours. Remove the pot from the oven and add the carrots and sweet potatoes.
Cover and place back in the oven for about an hour more, or until the vegetables are cooked, the broth is thickened, and the meat is tender but still holds its shape. Taste and adjust seasoning, if necessary. Serve the beef stew warm or let it come to room temperature and then store in the refrigerator overnight or until ready to serve.
This beef stew improves in flavor if made at least 1 day ahead. Reheat, covered, over medium heat, or in a 350 °F oven. Garnish with fresh parsley, if desired. To serve, place the stew in a soup bowl, garnish with parsley, and drizzle with olive oil if desired. Enjoy!