In a large skillet, melt the butter over medium heat.
Add the thinly sliced onions and bring onion to a simmer.
Stir the onions often.
Cook the onions 15 to 20 minutes until translucent. The longer you cook the onions, the sweeter they will be. Watch and stir the onion often. Do not burn them.
Season with salt, pepper, Italian seasoning, and Worcheshire Sauce
Add the flour to the onions and cook 2 to 4 minutes.
Whisk in the beef broth and bring to a low simmer.
Add the Thyme and Simmer for 20 minutes.
Preheat the broiler
Ladle the soup into four soup bowls.
Put the soup bowls on a baking sheet.
Cover each bowl with two slices of cheese.
Add 3/4 cup croutons on top of the cheese
Next, add a piece of cheese on top of the croutons.
Put the soup under the broiler just until the cheese melts.