Cut the beef into 1-inch chunks. Dredge the beef in the flour until all of the beef sides are covered. In a large soup pot, melt the butter over medium high-heat. In small quantities, brown the beef on all sides, removing the browned beef and setting it aside in a large bowl as you go. When the beef is browned, add the onion and the garlic to the pot. Sauté the onion and garlic for one minute, scraping the bottom of the pot as you stir. Return the browned beef to the pot and add the beef stock, pumpkin puree, Worcestershire sauce, salt, and pepper. Stir well and bring to a boil. Once the liquid is boiling, reduce heat to medium-low and simmer for 3 hours, stirring occasionally. Add the potatoes, carrots and thyme sprigs to the pot and increase heat to medium. Cook for an additional 30 minutes. Salt to taste and serve.