Warm the oil in large heavy pot until hot but not smoking. Add cornstarch and season salt to large ziplock bag and shake the meat in it. Brown meat on all sides for about 5 minutes. Transfer browned meat to a plate.
Add the onions and butter to the pot and stir over high heat until the onions start to soften, about 5 minutes. Reduce heat to medium and sprinkle in sugar. Continue to cook the onions, stirring occasionally, until nice and golden brown, about 15 minutes. Add the flour, thyme, and carrots and raise the heat to high. Stir for 1 minute, then pour in beer, letting it come to a vigorous boil. Stir in broth and tomato paste and return to the boil.
Return meat and accumulated juices to the pot. Let the liquid come just to a boil, then reduce heat to low. Cover and simmer 1 1/2 to 2 hours. Check on occasion and add beef broth if it is getting too dry. Season with salt and pepper to taste.