Heat the olive oil in a large Dutch oven over medium heat. Add the onion and sauté for 1 minute. Add the beef cubes and cook until browned, about 7 minutes.
Add the mushrooms, 1 1/2 cups Tannat, garlic and bay leaves. Add salt and pepper, cover, and simmer the stew over medium heat for about 1 hour, 15 minutes. Uncover and check the liquid level occasionally, adding 1/2 cup of water (or Tannat) if the estofado looks dry.
The estofado is fully cooked once the beef is fork-tender. Mix the flour with the remaining 2 tablespoons of Tannat until it is smooth and free of lumps. Add to the simmering estafado, stir, and continue to simmer for another 5 minutes so that the stew thickens. Add the Maggi seasoning and stir to combine well.