Place the butter in a large pot under medium heat. When the butter has melted add the onions and fresh Thyme. Saute the mixture for 25 - 30 minutes until onions are tender. Remove the fresh Thyme from the pot and add the flour to the onions and stir it into the mixture. Add the garlic powder,beef broth, sugar,kosher salt and beef base to the pot. Stir to combine. Simmer the ingredients for 45 minutes. To Serve:Place the soup in an oven proof soup bowl, top with croutons and grated swiss cheese. Broil until the cheese is melted and bubbly. Salt and pepper to taste.