I know that there are many ways to make this... However I like simple, and this gets the job done and everyone I have served it to has enjoyed it. In fact, my aunt will not eat it anywhere or anyone else except mine. I hope that you too fine this enjoyable and easy as well.
Slice onions thin in rings or if you like a little more body slice them a little on the thicker side. Sauté onions in butter in pan over medium heat of range top until golden brown, about 15 to 20 minutes.
Transfer Onions and butter to a pot and add beef broth, water, worcestershire, salt and pepper to taste... If you notice that the beef stock is bland add a dash of beef granules or a beef bullion cube (I use paste). Sometimes the broth or stock can have a milder flavor than I like I just fix it to my liking. Simmer on med heat for 10 minutes and turn to low for 20 minutes. While waiting on the soup slice your bread and toast it well toasted in a toaster. Be sure not to burn it, but you want it hard and crispy.
Heat your oven on broil. Some of the onions should have broke down a little that is completely normal and gives the soup the flavor you want. Ladle the soup into 4 oven safe soup bowls. Place a slice of bread on top of each bowl topping that with 2 slices of the provolone cheese. Place under a oven broiler until top is brown and bubbly about 5 minutes. Garnish with parsley and Enjoy!